|I doubt you can boot into Windows safe mode but you could try by tapping F8 at start up. |
I am thinking that johnr is probably right about the version your key is for and the version of WinXP that is installed are different. Could be any number of variations. OEM home/pro, retail home/pro, corporate, etc.
You could play with this for some time and not solve the problem.
If you have personal files you need to recover then there are ways to accomplish that. One method is to boot to a live version of Linux and copy the files to optical disk or an external storage device.
Then re-install WinXP using your CD and key. I assume you do have a CD.
I don't believe you can perform an over the top (repair) installation if the versions in question are different. An over the top install does retain the My Documents folder and the installed programs but would require installing any missing service packs and updates.
There may be ways you can check which version you have installed. You should know which version is on your CD.
Hirens boot CD 9.8 has something called mini WinXP on the CD. You may be able to find out the version using that, or a live version of Linux.