Try right clicking Combat Arms > Properties > Compatibility Tab > Under "Compatibility Mode" check the box that says "Run this program in compatibility mode for:" > Select either Windows 2000 or Windows XP (Service Pack 2) > Click Apply, and presto.
If that doesn't work, a reinstall is always a possibility.
(I use Windows 2000, never had problems with it. You can always change it later. I'm running Windows Vista, recently bought my laptop, 3GB ram, etc.)