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1.Boot to the Recovery Console (and hope you have the admin password)
2.Type cd $ntservicepackuninstall$\spuninst
3.Type batch spuninst.txt
4.The screen will scroll around, saying files missing, access denied and also copying files about. Just wait for this all to finish
5. Reboot. I was able to reboot in normal mode
6. Re-install SP2 (You must do this or the problem will recurr at step 7). REBOOT
7. Install KB834707 from MS Website
You may need to create this disk using another computer, you should also google slipstream servicepack and look for the most generic instructions that aren't tied too closely to a specific CD burning package (unless you have that package). Naturally, you should backup critical data before installation.
If you can't get SP3 from there try this page: Windows XP Service Pack 3 Network Installation [microsoft.com]
However, Kaled's advice above is probably best. Searching the keywords he highlighted will give you a number of sites with step by step instructions.