Being 'nuked' will only disconnect you from IRC/Internet, it won't reboot your computer. What you have sounds like some sort of trojan, there are a number of recent trojans that are extremely widespread that tend to reboot your computer.
You need to run 2-3 good virus/trojan scans. See
this thread for some resources. You need to use more than just one because no ONE can detect all viruses and trojans. Use two virus scanners and then 2-3 trojan scanners. Also read the whole thread for some good tips on staying safe.
Make sure that you have all available Windows Updates from
http://www.windowsupdate.com - You should regularly visit this site for the latest updates. If you use Windows XP, ME or 2K then it's especially important you keep an eye on the updates for the coming weeks, as Microsoft will be releasing a number of updates for extra security. Speaking of security, check out the firewalls section of the previous thread I linked to.
Hope this helps :-)
Regards,