Lösningen...
Jag hittade själv detta på webben:
"Error "Severe..." is Displayed
When I try to install TrueSync Desktop* or TrueSync Plus* on my Windows NT* system, the following error occurs: "Severe - This installation has detected that a previous installation or un-installation requires you to restart your computer."
Resolution:
Try the following:
Restart Windows* .
In your C:\Windows or C:\WINNT folder, look for a file named WININIT.INI. If you find it, rename it to WININIT.OLD.
Try to install TrueSync Desktop and/or TrueSync Plus again."
Det funkade!