I came across the error "SignTool Error:ISignedCode::Sign returned error: 0x80880253 The signer's certificate is not valid for signing." when trying to build/publish my VB code in Visual Studio 2005. I know a few people in the lab also use VS2005, so I'm posting how to get around this error in case you come across the same thing.
The first time you publish your solution, VS creates a certificate that expires in one year. If you try to update your solution or re-publish over a year later, you will probably get the error mentioned above. Microsoft explains this problem and gives a fix for it here... http://support.microsoft.com/kb/925521
I did not get the Microsoft fix to work for me, but I was able to work around it as follows.
Go to Project -> Properties -> Signing Tab.
Click "Create Test Certificate"
Enter a password
This doesn't exactly fix the problem, but it extends your certificate for another year so that you can publish your project.