What do you need a lighter version for?
You cannot trim Windows 7 to be lighter than it is as it is already the lightest Windows ever. All services that aren't absolutely needed at startup are set on Manual fast-triggers. If you remove any you will have unintended consequences. If your GPU cannot run aero it won't be turned on.
For low resource machines the only old-school trims that maintain are to edit Visual Effects of animation, sliding, fading and dragging intact.
Upgrading hardware is the way to go. Max out your RAM, then if you want better performance upgrade your processor and/or mobo. Or buy any new computer for only slighter extra cost. The weekly Sunday ad $350 special is more than needed for most users once you Clean up factory bloatware.
Gamers should look at the blazingly fast new core I5's or I7's.