• Right-click on Computer icon in Start Menu, then Manage.
• Open up Local Users and Groups node in the left tree.
• Find Administrator user, double-click it to open its properties
• Uncheck Disable account checkbox, press OK
• In case you have forgot the password for Administrator user, right-click and select Set Password menu item to change it

Now you should be able to choose Administrator account on Logon screen.

Still the same... Do I have to uninstall it before doing the manual install?

I don't think uninstall is needed, but you may try.
You can also try to make a Windows Repair. Maybe this will give some health to .NET Framework which now appears to be in not ideal state.

I installed XNA Game Studio successfully but ipi Motion capture is still crashing... I am out of ideas.

Me too :( Clean Windows installation is a last resort. Maybe you have an excess HDD to test it?

I reinstalled Windows 7 and now it works fine. More problems started to build up because of XNA Framework so I decided to get rid of them. So no problems anymore. Thanks for the help!

Nice to see you're OK. It's pretty long time since then :)