You also considered completing the following:
1) From the command prompt: (start->run-> type: cmd and press the enter key) CHKDSK c: /f You will get a message saying something to the effect that the system can not lock your hd, do you want to run CHKDSK the next time you reboot. Press Y Then reboot. This command will check your drives for problems/errors and attempt to fix them.
2) Once #1 is complete and your pc has reboot, download a program called System mechanic. It's fully functional for 30 days. Install and run it. Here you can do a number of things. Such as: clean out un-needed files like .tmp, .bak, .log files. It will also clean your system registry. You can even delete cookies and clear IE's history and offline files.
3) After you complete #2, download and install a spy-ware remover, such as Ad-aware. Install and run it.
4) Delete any programs, text files, data you don't use or need. IF you have a cd burner, burn these files to a blank cd for future reference.
5) Defrag your HD. If you've never done it before, it may run awhile, depending on the size of your HD, CPU speed and the amount of fragmentation.
6) Finally download and install (or buy) a good anti-virus program. Norton has a good one, along with MCAfee. There are otheres.
You should perform steps 1-6 (defrag, delete spy ware, do a virus check and update your virus definition files, delete un-needed files, etc) on a regular basis. Depending on how much use your pc gets, perform maint. at least once a month. If you use your pc a lot you may want to do maint. on a weekly basis.
This SHOULD help get your pc performance back to where you think it should be. Installing another OS won't solve your problems as each Windows OS needs regular maint. Also be sure to download and install any and all patches and upgrades while you are at it.