Need help solving an issue...Office 2007, Windows 7

Within the past two weeks (and never before) I am running into a problem in Outlook when trying to open for inspection any excel attachment that I have modified within the email, saved, and then tried to reopen to ensure it is right before I send it. I get the below message, when I try to open the spreadsheet:

"Excel cannot complete this task with available resources. Choose less data or close other applications."

As mentioned this is a problem that has just started to occur. I have an older computer, Dell latitude D630 with 4GB RAM, which I believe should be sufficient. I can only solve the problem by restarting everything, which is a huge time drain. This allows me to open perhaps two or three attachments before I then run into the same problem. Ideas?