Explorer Exe Wont Load After Reboot

I leave my computer on 24/7 but lately every morning when i wake up I'd come downstairs to error messages related to virtual ram running low (but a meter would read my actually ram used at 1-4%) Programs would crash due to not having enough memory and nothing would be running when i woke up. The weird thing was, there weren't any new programs that were running, it was all the same things that worked fine before. It started suddenly, and after updating a program called Azureus it seemed to worsen, but it was always the first to close. I noticed that Norton 2003 auto protect was disabled and it wouldn't enable so i upgraded to 2005 and updated it. It then found 30 some viruses, all being Trojan related. I think all of the files were related to TASKMNGU.exe and a file called something like MSMSGI.exe. Norton couldn't clean them so i rebooted in safe mode and scanned, when it found all of them it quarantined a few, deleted about 20 and 4 were left unable to do anything with. i then rebooted into normal windows, and was going to see what was left, and how to handle them, only when i clicked on my user, entered my password, and waited for my desktop to load, it never did. It froze with my background image there, all my processes seemed to start up except explorer (windows explorer, not IE, i use Firefox anyway). I tried rebooting in safe mode and had the same problem. I tried going to new, and running explorer.exe from task manager but it says i cannot find the file. I hit browse, found it and selected it but again it said it couldn't find it. I then tried repairing the version of XP with the install disk from Dell but it just takes me a command prompt, which im not sure what to do with. does anyone have any ideas how to get explorer to load?