Some unhelpful ideas:
1. Reboot your PC, keep trying until you get tired
2. Wipe your OS and re-install everything, if the problem persists, follow step 1
On another note:
I have windows 8.1 too, occasionally I get the iw3mp.exe error it normally has to do with a new map being loaded or some rendering failure with a particular mod. There are many factors that contribute to this instability, sound card, gfx card, drivers, directx, overlay software (e.g. xfire). I did not have this issue on Windows 7 x64 if that informs you in any way. I have an ASUS P5 Q Deluxe with RealTek Audio that I used for the longest time before switching to Creative X-Fi Sound Blaster MB PCIe. Anyhoo, this game is old and directx 9 is becoming a legacy with time. At some point with newer OS this game will completely start breaking down.