In Windows XP, right click on the
My Computer icon and choose
Properties. In the dialog box that opens, choose the
Advanced tab. There will now be a
User Profiles button. Click that and you should be able to choose your user's profile from a list. Click the
Copy To button. On the dialog that opens, there's an option to give other people access to log on to a profile. You have to copy it back and forth a couple times and you need to own two user accounts, but you can use this to tweak a user's profile without knowing their password.
First, copy the other user's profile to the folder your alternate user account would use, taking care to give that account access to log on the profile. Log on with that account and do whatever you want to the profile. Then, log back on with your original account and delete the user's original profile. Now copy the profile changed by your alt account back to it's original location, taking care to give the original user access to the profile again.
If this is for a new computer setup, where the original user has never logged in, you can do this copying your nicely configured profile over the default profile.
It's worth noting that this answer is only any good because you tagged the question
windows-xp. The process no longer works on Windows 7, as there are some registry settings that are not handled very well by the copy profile process. Rather than updating the Copy Profile process, Microsoft chose to simply disable the Copy button. You can find hacks to re-enable it, but none of the fix the reason the button was disabled and so they should be avoided.