whats your hardrive? also, have you run a spyware check, how many processes do you have running in the back round. Have you tried disabling your virus scan software.
Im leanin twards a HD problem, due to morrowwind+OS and Background Processes using more than a gig of ram, it has to use the HD to swap textures and such.
With the processor that you have its unlikely, but possible, that the virus scan program would be causing the problems, however if this is the case, its once again, your hardrives fault.
Good Luck