Your description of the problem is just vague enough to not be very helpful.
Assuming that you don't dual/triple/whatever boot your PC you should only have one operating system installed.
It seems from your post that you can get into windows just fine on subsequent restarts so I'd check the following things.
Examine your boot.ini
Typically it looks like this:
multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Microsoft Windows XP Professional" /noexecute=optin /fastdetect
Check your event viewer for warnings and error messages. If you're lucky and you have a misbehaving file that reports and logs entries into the event viewer you may be able to more easily identify the culprit.
There are dozens of things that could be wrong but try some of the built in windows maintainence tools like checkdisk/scandisk/defrag and if you get no joy then post back for more tips.
Also try to use spinrite 6.0. You can buy it and return it for a full refund if you don't find it helpful. You can also borrow it from a buddy if you have a buddy that has it. The owner of spinrite just asks that if you use it and it works for you then pay him for a copy.
If all else fails restore your last image backup (You do image your computer dont you?
At first my computer had Windows XP Home installed. I recently bought Windows XP proffesional and i also bought a new hard drive (it is a 120gb Seagate). I do not know which one of these caused this problem but anyways...
I installed both XP proffesional and the hard drive normally and followed all of the instructions provided. Now when i RESTART my computer it displays an error saying no operating system was found. If i completely shut off my computer and turn it back on everything works fine.
I have also noticed that if i access C:\Documents and Settings i have a folder for all of my current profiles along with the username i had before i installed these two things. When i view the properties of this folder it says the size is 0 bytes and contains no files. If i try to open or delete the folder it says "C:\Documents and Settings\Owner is not accessible." "Access is denied".
I would appreciate if someone could tell me how to fix this. If you need any other information on my system just ask. Thanks.