Hi Judy,
From previous experience this could be one of a few things. Please see below:
1) your computer recieved a device driver update via Windows update (possibly for the Graphics card) which is supported by Microsoft but maybe not by your Graphics card. Go to the Graphics card manufacturers website and look up their latest drivers and use them in preference to the Microsoft ones.
2) Your computer recieved an update from Microsoft which is non Hardware related that has caused it to have a problem. Annoying i know, but can happen. Check to see if there are any more recent updates.
3) Your computer hasn't been updated and there is something attempting to use a driver/piece of installed software in a way which causes a non patched system to fail. I know this basically couters anything said in point 1 but it is still a possiblity...if it aint one then it could be the other.
4) Your graphics card may have a problem. Working in shopfloor IT, i have found that, recently, a lot of our more 'budget' deployments are developing issues with a certain capacitor on the graphics card. this results in the capacitor litterally 'exploding' which causes an unstable system when running in normal mode...but strangely not in safe mode. Could also be the case on the motherboard i suppose. See this search for examples of what i mean:
http://www.google.co.uk/images?hl=en&source=imghp&biw=1276&bih=591&q=blown+capacitors+graphics+card&gbv=2&aq=f&aqi=&aql=&oq=&gs_rfai=
5) if it is likely that your computer has recieved a bash recently, it may be worth getting one of your more technically minded friends to check that everything internally is seated correctly. For heavens sake, try and avoid the PC retailers as they are likely to charge an arm and a leg for something which should not cost more than a cup of tea/coffee whatever your preference.
6) The RAM may have a problem. This is the hardware which the CPU uses as a buffer inbetween itself and the motherboard. It needs this buffer as it 'thinks' and 'outputs its thoughts' far much more quickly than the motherboard can listen to them. If there is a problem with it, it may cause Windows to splutter. If you have any technically minded friends, it may be worth suggesting to them to download and comit to CD/DVD something similar to the following bootCD utility suite to test the RAM:
http://www.hiren.info/pages/bootcd
7) The CPU Cooler may have come away from the CPU (i.e. the brain of the computer) causing it to overheat in certain situations particularly if there is extra strain placed on it. This tends to happen with the Cooler units provided with the CPU over time as they often use cheap plastic feet which are pushed through holes in the motherboard and then expand to hold the cooler unit in place. These can degrade over time and in some instances snap clean off. Lack of contact basically results in inefficient cooling and therefore 'brain' overheat. It may be possible to show whether there is a problem with the CPU Cooler from the BIOS system. They sometimes have a 'PC Health' menu which shows the current speed of the CPU Cooler Fan and current temperature of the CPU. If the CPU is Very hot and the fan is spinning really fast, this is just as bad as the CPU being really hot and the CPU Fan not spinning at all.
Your comuters HDD may have a slight or major corruption. Not sure about Windows 7 but in Windows XP, if you navigate through 'Computer' to show the 'C-drive' then right click it to navigate through to 'Properties', there should be a tools tab presented. Click this Tab then select the 'Check Now...' option in the 'Error Checking' section. Select the options to 'Automatically fix errors' and 'Scan for and attempt to repair bad sectors'. The computer will likely require a reboot at which point it will run through a system check to verify the HDD on the next boot up. If it finds any errors, they should be listed in the Windwos Event Logs area under an event labelled 'WinLogon'. Failing that there is a command line which works for Windows XP...again not sure about Windows 7...that can be typed into the text box within the start menu. Assuming that your computer is configgured in a standard manner, typing the following should do the same thing:
Chkdsk C: /R /X
...just type in the letter 'Y' to the prompt you may be presented with and press enter to continue, then restart your PC.
This may take some time to complete. If, on return of your system, the disk reports 'bad sectors' it would be advisable to use the system as minimally as possible but download and install:
http://www.snapfiles.com/get/easustodo.html
Run it to make a backup of your entire HDD, make a copy to a CLEAN/BRAND-NEW large format USB stick (16GB should be good enough) and switch off your computer upon completion. Head online (using a friends computer or somthing) to somewhere like http://www.dabs.com (or your reigonal equivillant) and order a new equivillant HDD...or a slightly better one if you like. If it's a Windows 7 PC, I'm guessing that it'll likely be a SATA II 500GB(?) HDD.
9) Could be that one of the expansion cards has an issue. I would not advise you as a novice to do this but if you open up your PC's case, there may be a number of 'cards' plugged into the motherboard. One of these would include the Graphics card. Others may included a USB expansion card, a network card a sound blaster audio card...basically, for anything that you may wish to do extra to the standard offerings of you motherboard, there is likely to be a card for it. A starter for 10 would be to check that they are all 'seated' correctly. By this i mean check whehter they are fully plugged in and are not standing proud of their socket. The next operation would be to take them all out (except for the graphics card) and see how your PC reacts without them in. If its better, plug them back in, one by one to see which one is causing an issue. Obveously, it would be best to call on the favours of a more technically minded friend to assist here....perhaps an offering of dinner may be a suitable payment this time 
10) Could be an-other issue. Again, it may be best to call on the services of a more technically minded friend. It may be a virus or other Hardware type issue. The Hirens bootCD I informed you about may have some more advanced tools on their which may help you to find the answer.
Hope this helps!
Your trusty Weedfreer!