I'm pretty sure you don't have enough RAM or your video card isn't good enough.
To check this out, go to Start-->Run--> and type DxDiag. Then, you can check your RAM, and video card.
This is the System Requirements (You should have the same or more than the recommended, or at least close to it, to stop lagging.)
Windows® 98/2000/ME/XP
Pentium® III or greater
256 MB RAM (512 MB RAM recommended)
32 MB Direct3D compliant video card and hardware T&L (i.e. ATI Radeon or Nvidia GeForce 2 or better. Geforce 4 recommended)
DirectX 8.0 compatible sound card
56.6k (or faster) Internet connection (broadband recommended)
16X speed CD-ROM
1.5 GB+ hard drive space
That has both the Recommended and Least.
Good luck.