another tricky question

...I recently purchased a s/hand comp... it came with XP Pro installed.
... I tried to install my ATO program ,but because of security issues , I need to update XP with SP1 (SP2)...
...I downloaded SP1 , but cannot install it because of dubious copywright issues with my XP operating system...(apparently the guy who sold it to me should have a wooden leg , a parrot on his shoulder and say 'ahhhhrrrr' a lot...)
...any way around this issue ?!!