You are a little low on RAM if you have the graphics setting high. Make sure you have plenty of HD cache space. Check all your drivers (esp sound.) As I recall it's Soundmax on that board. Reinstalling game with no mods would be a very good test to try. If you have tons of auto start programs and some are spys like Kazza, your 1 meg might not be enough to run the game. SH4 is much more demanding than any other game I have ever played. If you have spies, you can download a free killer from
www.tennebril.com. Do not use adaware or other donationware. They do not get rid of the worst spies and make things worse.
Take a look at START->Run->"msconfig<enter>" Startup tab. Uncheck anything you don't need to run your PC. You can recheck things if you have trouble after a reboot. If you're going to run on a 1 megger, you really need to have a squeeky clean memory on bootup.
-Pv-