There are some steps to try BEFORE reinstalling the OS or formatting the drive......
Reformatting your drive will erase everything on your drive (well not 100% , but for a normal user.... it gets rid of everything on your drive).
After formatting a drive u must reinstall everything.. the OS and all of your drivers and applications.
Reinstalling an operating system instead of reformatting is a good idea if you have somthing wrong with the operating system itself.. but before that step there are are a few things you can try...
For example XP has the system restore (symantec's GOBACK works better) there is also the Recovery Console and the Automated system Recovery.... THESE three things should be attempted BEFORE reinstalling an operating system... and if all else fails format your drive... (try searching on the internet for information on "fdisk")