Do you mean a password is required before you can even boot Windows
(BIOS password) or a user account password (Windows)?
If the former, go into the BIOS (providing the necessary password as
prompted) and remove the password from there. It is usually under the
"security" section but differs from BIOS to BIOS. If you didn't put the
password there, ask the person who did.
If the latter, see below (works for both XP and Vista):