Slow Opening of Emails with attachments

We run a Windows 2000 Domain with Exchange 5.5. Over the last few weeks we noticed that emails with attachments are taking a longer time to open for the users. They are all running Outlook 2000 as their client with all the latest Service Packs done. No changes have been made to the email servers. We notice is more with adobe documents, but it also happens with excel and word documents.