VS 2010 Can't open any forms in design mode

I have just upgraded from Vista HP to Windows 7 HP and in my first use of VB 2010 Express since the upgrade I find I can't open any forms in design mode. When I try I get the error "The designer loader did not supply a type resolution service, which is a requirement for CodeDom serialization." I've googled this but found no solution. Does anyone here know how to tackle this, please?

I should add that the problem isn't confined to my current project - it also occurs if I start a new project.