My game keeps lagging. Help!

First off, my specs:

Pentium D 2.8GHZ
2GB RAM
Nvidia 7300 GS 512 MB Vid Card
About 3GB left in Drive C where game is installed

All my settings (even the application-specific settings in my video card) are on the lowest possible, yet the game still lags a bit:
1. After the jump ball when the player gets the ball
2. When a player shoots a ball and people rebound
3. When there's a lot of action going on (cut scenes also)

What can I do to remove all lag from my game? My friend's PC (Pentium D 3GHZ 2GB RAM Nvidia 7200GS 256MB Vid Card and almost same amount of space left in Drive C) can run the game without lag at all, and he has it all on low specs.

I don't care if the game looks bad, as long as it runs smoothly.

heat problem??

Your video card is quite weak, and you may be running out of space on your hard drive, you typically want more than 3GB remaining, that will slow it down.

3GB left, dude, delete some of those you know what *wink*..hahaha...but yeah..I have about 100gb left and noticed my cup slowing down already...

Nah, none of those *wink*
It's hard cause my C drive is full of my programs, and I don't think I can delete any of them... Do you think it will work if I install my game on an external hard drive?