Assassin's Creed

As soon as i press the game's launcher to start the whole computer becomes slow ,then i start a game the loading in the animus and on every loading it takes forever ,now in game the game lags every 10 seconds and keeps lagging and the game takes forever to load ,now i know this isnt a performance issue because i could run this game before on highest settings without a single lag ,i uninstalled it about 3 months ago ,now reinstalled it and it runs like **** ,and btw it runs on the highest settings and the lowest settings but with the same lag timing every time and this proves that its not a performance issue ,so how i can play it normally again like before ???
my system specs are :
Windows XP sv 2
Nvidia Geforce 8600 GTS graphic card
Intel Core duo 2 cpu
2 gb of ram
E4500 @ 2.20 GHz
ive got direct x 9 and ive got the latest graphic card update

Additional details - 10/19 11:57AM

It seems the problem is with my drive , i did a reinstall on a different drive , and the seemed to solve the problem , now the game runs sweet like before,i installed it the first time on drive f ,and it was slow , i uninstalled and reinstalled on d and it worked try that , and btw make sure nothing is running in the background ,keep the anti-virus and stuff but dont use any program while it is installing
peace , i hope this will solve your problem =)