Problems Playing Original F.E.A.R new system.

Hello People,
I’ll start with my basic system specification which is
AMD AM2 dual core processor 4200
Nvidia Geforce 7600GT
160GB SataII Drive
550W Power Supply
1024 DRR2 533mhz Ram.
I recently downloaded the new F.E.A.R extraction point game demo and ran it maximum settings from which it ran absolutely fine. Because I never owned the original game I decided to buy it and give it a go. This game is now over a year old and because my system handled the recent demo (september2006) I thought it would play the original game (came out start of 2005 I think) no probs on full settings. When I tried the original game I get quite a bit of jerking and like lag and it doesn’t run smoothly at all. I play games like Half Life 2 and Doom 3 on max settings all the time and don’t have no problems so I think it’s a software problem. I have the latest graphic card drivers 91.47 nvidia and have also updated the original F.E.A.R game to version 1.08 to see if that helped but it didn’t. The only hardware I’ve changed recently is I added an optical mouse! The only other thing I noticed was when installing the game it said it needed to install direct x 9c(august) and wudn’t proceed unless I clicked yes. Im pretty sure I had a newer version of direct x already so I don’t think it installed it anyway. Any suggestions? Im gonna play my copy of half life 2 2nite and see if that’s still running ok,then I’ll try re-installing F.E.A.R see if it makes a difference.