When I do an update this is the message I get after it shows it is downloading:

The digital signature on the update is missing or invalid. The update may have been tampered with since being signed by Apple. The update will be downloaded and checked again the next time that Software Update runs.

What version are you updating?

Thank you for responding! I have it fixed now. My version is Mac OS X 10.6.8. I was experiencing various problems when my computer died. I thought it might be due to a needed update, but it was just an adjustment to the date/time setting. One little thing caused a big frustration.

Thanks for your help!

Date and time will cause a bunch of problems good to here it is fixed