Open Registry:
HKEY_LOCAL_MACHINE\SOFTWARE\Stardock\
HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Stardock\
Delete any WindowBlinds reference in either key (the second key will most likely be the culprit).
Re-boot back into Windows. Try upgrade.
Failing that:
Install Window Blinds 7, re-boot back into Windows. Try upgrade.