I have a signed executable. The certificate for that signature says it is usable until 2007, still Windows (7 Professional) says it is valid if I'm opening it via properties dialog.
How is that possible?
The whole point of a signature is that it is permanent, irrevocable, and undeniable. Provided you can prove that a signature was made during the validity interval for a certificate, you should accept that signature as valid.
You are correct that you cannot trust that certificate anymore to make new signatures because it could have been compromised, unless it was revoked, you should assume that it was trusted during its validity interval. In this case, you can prove the signature was made during that validity interval (because the signature is timestamped by an authority you still trust and the signing certificate was never revoked). So the signature is forever valid.
Whether a certificate is shown as 'valid' or not depends on whether it is valid in the context in which you are viewing it.