The possibilities are legion You can try the recovery console from the XP
CD, but that gets pretty esoteric.
These instructions for "Last Known good configuration" are from the XP Help
file.
"To start the computer using the last known good configuration

Click Start, click Shut Down, and then, in the drop-down list, click Shut
down.
In the Shut Down Windows dialog box, click Restart, and then click OK.
When you see the message Please select the operating system to start, press
F8.
Use the arrow keys to highlight Last Known Good Configuration, and then
press ENTER.
If you have a dual-boot or multiple-boot system, choose the installation
that you need to access using the arrow keys, and then press ENTER.
 Notes

Choosing Last Known Good Configuration provides a way to recover from
problems such as a newly added driver that may be incorrect for your
hardware. It does not solve problems caused by corrupted or missing drivers
or files.
When you choose Last Known Good Configuration, only the information in
registry key HKLM\System\CurrentControlSet is restored. Any changes you have
made in other registry keys remain.
NUM LOCK must be off before the arrow keys on the numeric keypad will
function."

Recovery console is also covered in the XP Help file.
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****************************************************
My father, who has had virus infections on his computer in the past,
currently has a major problem with his computer. When he turns on his
computer, he can get to the log-on screen and then no further. Any attempt
to log-on sees the computer start to load his settings, then swiftly return
to the log-on screen. When we go into DOS mode, we can access the floppy
drive, but cannot change to the hard drive or CD. CDs will not automatically
run. As software comes on CDs these days, we are therefore unable to run any
anti-virus or diagnostic software. He has Windows XP and has not loaded any
new software or devices in recent months. There is therefore no obvious
trigger for this. In addition, he uses the Internet but not email. We have
searched the internet and specialist magazines, but have not found any
reference to this problem. The computer seems to be completely locked.

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