By "reboot" I'm assuming you really mean reinstall Windows? First of all, it will help if you specify the version of Windows you're running: XP, Vista, 98, 2000. Chances are you've got registry problems and/or a whole bunch of spyware/adware lurking on your computer. Try these things first before you reinstall Windows (all are free and I've provided links for all of them):
Download and run
CCleaner - especially, the registry cleaner part.
Download and run
AVG free - this will eliminate any viruses you may have gotten infected with.
Download and run
AdAware Free - this will clean out most/all adware you may have gotten infected with (download the Ad-Aware 2008 Free version, not the Plus version).
Also download and run
Spybot Search & Destroy - this will help get rid of any spyware you've got floating around on your PC. Get it from the link I've provided, there are some imposters out there.
Finally, if you're using Internet Explorer for your everyday browsing, switch to
Firefox 3 instead. Internet Explorer is one of the primary routes viruses and spyware use to get onto your PC. Internet Explorer is needed for Windows Update, but you shouldn't use it for everyday browsing unless you know what you're doing.
Some additional advice: Don't open attachments in email; download them and scan before opening. Turn off HTML display in your email client. Don't install screensavers or "free" software unless it's from a trusted source - and even then be suspicious. Run Windows Update (in your Start menu) regularly - say, once a week - and make sure you install all critical security updates (Windows Update does use Internet Explorer - this is about the only time I use Internet Explorer when I'm on a Windows machine).
The above is a solid day's work but you should be doing all these things even if you do have to reinstall Windows.