I am having this problem on a Dell T3500 running Windows XP (SP3). A reimage solved the problem for a few days, only to return. The freezes were accompanied by video corruption. The video card was replaced and the problem ceased for a couple months. It returned last month, and after 4 hours running the latest Dell diags and BIOS version, painstakingly disabling and re-enabling startup items in msconfig, I noticed a quirk - whenever the wireless mouse was attached the problem returned, but when the standard wired mouse was used I could login and work normally. It's worked normally for a month and the problem has returned. I am completely mystified because Dell diags show all tests pass. chkdsk doesn't find anything weird, and the Event logs are relatively clean. Anti-virus and anti-spyware are current.
This problem is similar to http://www.experts-exchange.com/OS/Microsoft_Operating_Systems/Windows/XP/Q_27562314.html