Reformatting and starting over is a pain in the **** and takes a day or so, so avoid it if you can. I do know people who do it every six months or so anyway, on the basis that bad stuff gets onto your computer no matter how careful you are and the only way to be sure it's all gone is to start over. I'm of that viewpoint myself, and go through this once or twice a year VOLUNTARILY! But I do make it easy for myself by ensuring I can reload Windows and all its updates as easily as possible, that I have all drivers readily to hand to reload, and that all my data is kept in one place so I can easily offload/upload it.
Don't forget to scan any data you remove from your computer very thoroughly before (or as) you put it back. Or you could end up reinfecting your machine directly after cleaning it!
If you get more sophisticated you can, directly after reloading everything (operating system and programs) so you have a complete working machine, and before you've exposed it to possible reinfection on the internet, take a system backup onto an external hard drive. Then next time, all you need do is reformat the hard drive then restore your backup. You'll probably have made detailed running changes since the backup and you'll have to re-apply these manually, but it's an awful lot easier and quicker than doing everything by hand. I suggest you keep your data separately and not as part of the backup. I run three external hard drives to help me do all of this under control - they're so cheap nowadays I recommend anyone to do the same.
If you want advice on how to take a backup then PM me - available methods vary quite a lot depending exactly what OS you're running.
AmbergrisCaye.com 
