I have Dual processors 2.80 GHz, 3.00GB RAM Brand new HP Pavilion Slimline. The game boots fine and the menu screen is fine, I have all settings on low and it's lags like crazy from the birth scene. Windows 7 OS, any idea??

Sounds like just a weak system. It doesn't matter how new the system is...it always depends on hardware...and very rarely do slimlines come with anything special.

Please post a dxdiag so we can see full specs:
Start - Run (if Vista or Win 7, the search box acts like Run) - type dxdiag - hit Enter - "save all info" button - open txt file and copy/paste EVERYTHING here

Picture of process: http://www.windowsresource.net/cimages/gotodxdiag.jpg

what's your gpu?