Thats easy. press windows key+break, or right click my computer->properties. In the system properties, you would see like:
System:
Microsoft Windows XP
Professional(or home whatever)
Version xxxx
Service Pack 3... or 2 if thats installed.
If your SP is still 2, visit windowsupdate.microsoft.com in IE and run windows update manually. Or just download the EXE file for IT professionals and developers in microsoft.com/downloads. If you have SP3 already, no problem
And thanks for the info, I dont think most people in this forum use norton, but its always good to know stuff