Gaëtan;
Try booting in the Safe Mode and UNDo the changes.
From XP HELP:
[[To start the computer in safe mode
You should print these instructions before continuing. They will not be
available after you shut your computer down in step 2.
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 the appropriate safe mode option, 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
In safe mode, you have access to only basic files and drivers (mouse,
monitor, keyboard, mass storage, base video, default system services, and no
network connections). You can choose the Safe Mode with Networking option,
which loads all of the above files and drivers and the essential services
and drivers to start networking, or you can choose the Safe Mode with
Command Prompt option, which is exactly the same as safe mode except that a
command prompt is started instead of the graphical user interface. You can
also choose Last Known Good Configuration, which starts your computer using
the registry information that was saved at the last shutdown.
Safe mode helps you diagnose problems. If a symptom does not reappear when
you start in safe mode, you can eliminate the default settings and minimum
device drivers as possible causes. If a newly added device or a changed
driver is causing problems, you can use safe mode to remove the device or
reverse the change.
There are circumstances where safe mode will not be able to help you, such
as when Windows system files that are required to start the system are
corrupted or damaged. In this case, the Recovery Console may help you.
NUM LOCK must be off before the arrow keys on the numeric keypad will
function.]]
