Yer i had the same problem! it appears that after 30 days (if you have not activated XP) it boots you out and wont let you back on!
A Real Pain! However, i was able to boot up into windows in safe mode (F8 during startup) and then log in to back up any files. I then did a format and then re-intstalled XP!
I'm not sure why your copy of XP wont let you in even through safe mode, but it might even be worth trying a repair using your XP disk. This has helped me before!
Hope this helps! However, you may not have to format - it was the quickest and easiest thing to do and the time (and my computer was going slow anyway).
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