Login problem

I updated to windows seven about a month ago and no problems until this week. I turned on my computer and loged in, and it was extremly slow and unresponsive. When I say extremly slow, I mean it took over an hour and some of my desktop icons were still white. I tried Ctrl+Alt+Delete and closed every single open process it would let me, and still no luck. I booted in safe mode and turned off everything that started at login that wasn't a toshiba process. I have no idea what the problem is however when I first noticed the problem in the bottom right corner it said Windows 7 build 7600 invalid. Or something along those lines so I foudn the activation thing and activated it and now i don't see the message any more but im still having the problem. Starting in safe mode with networking is still fast though. I have a Pentium(R) Dual Core Processor with 3 GB of RAM running 64-bit operating system. Anyone have any ideas? Thank you in advance.

Alright so its not a memory error and the processors show up under the tab. So I was going to try antivirus, but it won't let me install it in safe mode? It gets to the preparing installation and stops part way through. Any suggestions? Thanks again.

I havent ever used an A/V. I tried when I got my first laptop 3 years ago, crashed six months in because of virus' so I havent used one since. And I assume by last good configuration you mean a system restore? Tried it but it won't restore back to a good time. I can try installing another virus software or something, or what would you recomend for when I get this sorted out anyways. I'm thinking im just going to have to do a clean instal of 7 and hope that takes care of it.

Alright housecall is runing now and i did a quick scan with the malware one, 2 infected 1 file one registry. So removed them and now im doing a full scan with it. It's not like I want to do anything drastic but I need my computer for classes so I need to try and fix it asap.