How to do more than ONE backup of OS Win XP

Good evening,
my computer is HP with WINDOWS XP Home edition.

1.When I bought the computer I did back up of Operating System.
Long time has been pasted, I tried to reinstall the Windows from the said back up, but I could not because either the backup was done wrongly or for some other reasons.

2.I tried to do new back up - the computer said me that only one copy (or back up) is allowed. So know I cannot reinstall the system from the disk with the OS backup.

3.What have I to do to bypass ( or to put off) the said limitation - only one back up is allowed. How to do more than ONE backup of the existing Operating System?

Thank you in advance
petb