A reformat is always a good idea because so much crap accumulates on your HD you wouldn't believe. Anyway the following assumes you have a CD with your OS on it.
What you need to do is create another partition on your HD. Then use the windows backup util: start->accessories->system utils->backup.
Backup everything you can fit INCLUDING the system state ( this backs up your registry). If you back up the folders where your applications are and the registry you can then do a restore and not have to reinstall them.
There are plenty of sites that tell you how to create new partitions - but using Partition Magic is the easiest...if u don't have that, use fdisk for older windows versions.
Once you have backed everthing up, you can do format c: /Q /NTFS or /FAT32 at a command prompt (put whatever your drive letter is in the place of c
. If you type format/? then it will tell you what the parameters mean.
It that asks you if you really want to format.
If you press yes, the current partition (c: in my example) will be quickly wiped. The other partition is not effected and your data on it is safe.
If you don't have space for a backup partition, then backup onto CD-Rs.
After this, just reinstall windows, start backup again and do a restore for system state (this restores registry among other things) and all your data.
It is a simple process but if you need more help search on the web for windows backup.
Ok I have to be honest, I've reformatted and reinstalled different versions of windows many times but I haven't tried windows backup because I chose to reinstall all my applications. What I said should work in theory as that's what it's designed to do: enable you to recover your system state.