|Reformatting would be a last resort for me. |
Before doing that, I would try the following in no particular order. I would run all of them (except the defraggers) because each one will find things that the others do not find.
If you do not have an antivirus program installed, download the free version of AVG and install it.
I would run SpyBot, Adaware, and Microsoft AntiSpyware (All are freeware).
Also, run EasyCleaner by ToniArts (freeware registry cleaner and other utilities) and run the registry cleaner. It is safe to follow his recommendations once it is run.
Get a copy of RegSeeker (freeware registry cleaner) and run it but ONLY delete the items in GREEN fonts. Deleting others can lead to problems. It is an aggressive but very good program.
Also, go to Windows Explorer and then right click on the C: drive (other drives or partitions too). Select "Properties" and then click on the "Disk Cleanup" button. Do this for all drives or partitions one at a time.
After this, either run the built in Windows defragmenter or download a copy of something like the free trial of PerfectDisk (It will run as a fully operating version for 15 or 30 days. Can't remember which)
Try the computer after this and see if it helps.
These things will take a while by the time you download any programs that you don't have and running them all but you will probably like any that you have not already tried.