I'm not a wizard but...have you tried booting into safe mode yet and seeing if it's just as slow? You typically need to press and hold F8 as the machine is starting to get to the boot menu that allows you to choose the boot options.
These articles are hardly good tutorials but they'll explain how to boot with as little as possible enabled and then reboot with more options/drivers enabled. Keep doing this until you see where the big slowdown occurs.
http://support.microsoft.com/kb/315222/
http://support.microsoft.com/default...b;en-us;316434
This article is more of a tutorial:
http://www.pcstats.com/articleview.cfm?articleID=1643
I once had a motherboard NIC on a desktop system go bad and when network was "started" everything went to hell and the PC started crawling. The network worked well, but had lots of failures and required a lot of packet resending.