Unknown publisher / cannot verify digital signature.

Since moving house I was getting numerous bugs with my vista 32bit pc which
included being unable to install any software (downloaded anyway). The
reason always centred on the digital signature not being valid/verfied.

I even resorted to reinstalling my pc to factory settings and now I can't
install anything from drivers to windows own msnm. I've tinkered with the
internet options after reading about possible solutions but either it
crashes, doesn't do anything or the signature is still invalid.

I have since moving changed to wireless internet so I don't know if that is
the problem (using a BT home hub) and used windows update but other than that
I can't think of anything I've done. I have looked at the digital signature
and it has a valid date from/to but apart from that I don't know what I'm
supposed to look at.

I'd be only to happy to provide any more information and I'd appreciate any
help.