If you have an OEM machine, your fastest way to a clean machine is to back up your personal data and uyse the factory restore, install your preferred AV, download the latest drivers for yoour computer from the OEM web site, install your 3rd party apps, and run Windows update repeatedly until you get no more updates. Now restore your personal data. If you don't already, get an external hard drive and make an image of your hard drive for speedy restores in the future. You could even use a 32 or 64 gig drive to make the restore image. This would be the the fastest restore method - or using a small SSD.
Manual steps:
If you have a 32bit version of Windows, make sure you have 4 gigs of RAM. If you have a 64 bit version of Windows, consider upgrading to 8 gigs of RAM.
Virtual memory: A little known and little practiced trick to speed up your computer is to optimize your virtual memory. Do a quick search of the net on how to access the virtual memory. There are many great pages for this out there and I don't want to recreate the wheel. (You have to be logged on as an Admin) Once you find it:
1. Turn off virtual memory on all drives.
2. Run the command MSCONFIG, go to the Startup tab, and deselect everything.
3. Turn off your computer and disconnect the network cable or turn off/eject the WiFi card.
4. Power on your computer and logon to an Admin account.
5. Run the Windows System Defragment tool. Now, when you create a VM file, it will be all in one contiguous file, making it much faster.
6. Access the Virtual Memory tool again, and enable it but deselect the option to Let Windows Manage the VMM. Now, assuming you only have one hard drive, create a VMM of fixed size, specifying the size as follows:
RAM less than 4gb : 2x the amount of RAM in your system.
RAM 4gb - 8gb: VMM = size of RAM.
RAM > 8gb ... you probably don't need a VM file but it won't hurt. Don't go bigger than 8 gigs, though.
note: If you have more than one drive put the VM file (aka Swap file) on the most used partition of the least used hard drive.
7. Save out of the Virtual Memory configurator, run MSCONFIG and go to the Start tab again and then re-enable only those items you want to start when you log on (be sure to include your AV application) and then reboot.
Most system degradation is the result of Malware/Spyware. Most AV/Spyware programs claim to do both anymore. YMMV.
First and foremost: given your lack of knowledge you must have a quality AV program on your system. My preferred free AV at this point is Quiho 360. It has performed very well in the independent test labs, better than most free and paid programs, and I find it much less annoying than AVG and Avast (two of the other better free progs). For paid AV programs, the top 3 at; the test labs a few months back were eSET, BitDefender, and Kaspersky (which I currently own/run). AV mfgs are constantly trying to improve their performance so these rankings can change, but you won't suffer if you use one of those 3.
Now, install and run Malwarebytes Free Edition and scan your computer. This is an ideal ondemand scanner (as opposed to the real time protection of your primary AV program). They also offer a real-time version which I am sure is quite exceptional, but I haven't seen it tested yet.
You should also run an anti-spy program. Some great free ones are Spybot Search & Destroy, Super-Anti-Spyware, and Adaware Free.
Use CCleaner (free) to clean-up your system of general junk.
For cleaning up other things, the Windows Repair All-In-One tool from Tweaking.com is a great tool. Another good one is IoBit's Advanced System Care (Free).
Using a usb stick for Ready Boost will slow your system down, trying to do what your virtual memory is already doing but using slower memory over an even slower interface than what your hdd already provides. (This assumes you aren't using the highest performing flash memory and USB 3.0 or Thunderbolt (or similar).)