If you don't know either the user password OR the admin password (check that administrator password isn't simply blank), then you need some form of password "recovery" utility, such as Trinux.
Trinux is simply a bootable CD with a stripped-down linux with various utilities, among them is the ability to modify the SAM database (containing the user passwords) on Windows NTFS system disks. Certainly works on XP and Windows server, though I've never tried it on Vista.
There are probably lots more similar (free) utilities, but that one I use!
Edit to add that you don't revcover the password, you simply reset it. Simplest is to make it blank.
SD