Evanelpus I wouldn't be too bothered about it being a pirate, if it is the OP is a dork for trying in the first place as he could download a trial version (time limited) from MS, as is the case if installing from original disk without valid product key.
MS FSX allows you to reinstall 3 times then reverts to above, at that point you have to phone MS. Someone once 'borrowed' my copy, I was unaware, and the thief got locked out even with my key. However I later tried to install on my new laptop and I got locked out, was on my mobile BB at the time, but when I got home and plugged in to terrestial ISP the key was recognised. This may have something to do with it, if OP has changed ISP.
Alternatively, and this is a guess, if he has gone through a major hardware change this could be the cause, like XP OEM reacts.
Pessimistically, the installation disk may have been corrupted in which case he either repurchases or contacts MS.