Maybe your Hard Drive access? I find when I reinstall windows 2k it changes my HDD drive access to PIO mode and I then have to change it do DMA.
It made a big difference with Logic Audio and loading loads of wave samples from hdd (I could use 16 separate channels for wave before it lagged with DMA enabled rather than 4)
I don't know but, laggy hard drive could drop the fps by not streaming the data fast enough and seeing as your computer is pretty old i'm guessing your virtual memory is working overtime
It made a big difference with Logic Audio and loading loads of wave samples from hdd (I could use 16 separate channels for wave before it lagged with DMA enabled rather than 4)
I don't know but, laggy hard drive could drop the fps by not streaming the data fast enough and seeing as your computer is pretty old i'm guessing your virtual memory is working overtime