No need to wipe and install everything. First off try this:
Open Start ---> Run
In the run dialog enter:
cmd
After that, you will have a command line. Enter:
chkdsk /f c:
It will tell you it cannot check the disk while windows is running (or something similar) and will offer to do a boot up scan instead - choose yes.
Reboot the machine.
Let it run through the disk check. Next, download the following programs, update, and let them scan your PC for problems.
Adaware SE Personal
Spybot Search & Destroy
CCleaner
you can run your favorite scanners too - I really like the above free ones though.
Next click on Start ---> Run
Enter:
regedit
Browse to: HKEY_CURRENT_USER ---> Software ---> Microsoft ---> Windows ----> Current Version ----> Run
Highlight all of the keys in the stuff on the right hand side and delete them (unless one is one you recognize as your anti virus software.)
Once they are all highlighted - delete them. (Right Click ---> delete) or delete key.
Next scroll way back up and browse to: HKEY_LOCAL_MACHINE ---> Software ---> Microsoft ---> Windows ----> Current Version ----> Run
And do the same thing.
There will be a key called "default" you can't delete - don't worry about it.
Exit registry editor.
Reboot your machine again.
See if it is running any better.
If it is not... then do the following (provided you have a windows xp cd handy)
Stick in the XP cd - reboot the machine (don't go through the options that pop up when you stick it in while running windows)
It will ask if you want to install or repair - select install (don't worry we're not reinstalling, but the first repair option dumps you to recovery console - and its tough and worthless without lots of training)
Eventually you will get to the partition manager - it will list your hard drives to install to. It also has the option of hitting "R" for repair here as well. Do this. This will refresh all your system DLL files. You will see messages about deleting files - don't worry - its deleting the dlls its getting ready to replace, not your files. It may ask for some driver disks along the way - if you have them good, put them in - otherwise hit cancel. You can install your drivers later if need be. This will run and look like a regular windows install but it is not. When it is complete all your installed programs will still be installed, all your settings will be intact, and all of your files will still be there.
Remember it is the 2nd option to repair, NOT the first. The first is confusing and unfriendly. Use the 2nd repair option on the screen where it lists the hard drives in your computer.
This will really get your computer into good shape, and you don't need to lose everything to do it.
But as always there is some risk so backup anything 100000% critical to you.