Ronkeuren
I'm afraid your PC in itself is pretty out of date for a modern game, so there's probably not a lot you can do to make the game go faster if you've already tried the various solutions that have been posted on here. In case you've missed any of them, try the following:
1) lock refresh rate to 60hz
2) disable sound acceleration
3) increase game priority
4) put detail on low and res on 640x480
5) close all other programs/processes
6) defragment your hard disk
The Geforce 4 is definitely better than the S3, but it's not a card good enough for many modern games. Your 700mhz Processor is the bare minimum for running a game, and 256mb of ram is nearing a bare minimum too.
The other person who got the game running on a similar computer to yours may have had a much better motherboard or faster ram, it's hard to tell with pc's. They may also have tweaked their system settings quite a lot, which you may not know how to do. The biggest difference is that they have a Geforce 3 card. There aren't any GF3 MX cards that i'm aware of, so despite your card being a GF4, its performance will be a lot lower because of the MX which basically means it's a budget card.
My personal recommendation would be to do an upgrade on your pc the next time you can afford it. Without changing the motherboard you could probably get a 1.2ghz P3 if you really hunt around which would keep you going for a while longer, as well as another 256mb of ram and a geforce fx card. All in all you could get the above for about £200 - £250, but the cpu and ram would still be quite old, so it may be better to save up and upgrade everything properly for about £500-£800