How to Clear the WPF Font Cache
PROBLEM
You may experience an issue with the Test Studio user interface. A corrupt WPF font cache may cause the drop-down menus to not expand and their values unable to be selected.

SOLTUION
Clear the WPF font cache.
- Start > Control Panel > Administrative Tools > Services (or type services.msc from the Run command box).
- Stop the Windows Presentation Foundation Font Cache 3.0.0.0 service.
- Delete the cache file: FontCache3.0.0.0.dat
- WinXP: C:\Documents and Settings\LocalService\Local Settings\Application Data
- Vista/Win7: C:\Windows\ServiceProfiles\LocalService\AppData\Local
- Restart the service or the machine if needed.
Please see these threads for more information: