When logging into SCTM, the drop down menu's are visible but not their content or Test Cases aren't visible. Below is an example of the issue:

If you are experiencing this issue only in IE8 and no other browsers, then you need to apply several Microsoft updates:

Cumulative Security Update for Internet Explorer 8 for Windows XP (KB2829530)
Security Update for Internet Explorer 8 for Windows XP (KB2847204)
Update for Internet Explorer 8 Compatibility View List for Windows XP (KB2598845)
Once these have been applied, make sure that all checkboxes in dialog "Compatibility View Settings" in IE 8 (Tools / Compatibility View Settings) are unchecked.

SCTM should now work as normal.