If you mean like this, right click on My Computer on your desktop (if you have an icon for it there) or go to
Control Panel > All Control Panel Items > System
from the start menu - bring up advanced system settings, click on the advanced tab, and under performance, click the settings button. Adjust for best performance - that will turn off all the rounded edges, etc, giving more of the classic XP look. You'll then want to go to personalization (right click on the main desktop screen, and choose the personalization option). Choose Windows Classic for your window color, and there you are. You'll end up with a screen looking very similar to the 2nd attachment, which looks very much like the original XP desktop, and, to a certain extent, the 98 desktop as well.
edit - add: yes, that is a Windows 7 x64 screenshot in both cases.