Apologies for the scattergun approach but hopefully one will get us closer to the problem.
You say the PC's are similar - is the hardware - ie CPU RAM motherboard graphics network card all the same?
Are all drivers up to date following the restore?
have you checked the programs in startup to see if there are differences.
Re attachments there may be a block on this either in firewall or other security program - Avast, a-squared, Ad-aware and Spybot you mention are not likely candidates for stopping attachments unless there's a realtime scanner blocking.
Are there a lot of mails and folders files in Outlook Express? Do you use the same ISP for all mails from all computers - presumably not the same email address on all three