This happened to one of our clients- a simple setting change fixed the probelm, but tracking it down was tough.  The browser was thought to have been corupted somehow- reinstalling IE8 and looking for a virus were not the culprits.

I got this from support Mozilla, the solution will fix IE as well.
http://support.mozilla.com/en-US/kb/Website+colors+are+wrong :

Windows settings

Windows has settings that can affect the display of web pages in Firefox.
High contrast

Ensure that Windows is not in high contrast mode.

1. If Firefox is running, exit it.
* From the menu at the top of the Firefox window, select File and the Exit menu item.
2. Open the Windows Control Panel.
* (Windows Vista) Click the Windows icon and choose Control Panel.
* (Windows XP) Open the Windows Start Menu, then choose Control Panel.
3. Open the Accessibility dialog.
* (Windows XP) Double-click Accessibility Options.
4. Click on the Display tab.
5. Make sure the checkbox beside Use High Contrast is unchecked.
6. Press OK. Start Firefox and see if the colors are now correct.

256 color mode