System Resources

I am running Windows 98SE with Office 2000 but when I am on the net checking my mail through outlook I keep getting a message telling me “Can?t open this item. Out of memory or system resources. Close some windows or programs and try again.” I normally only have Internet explorer, outlook and Norton anti virus running. I have 512 Meg of ram and thought this would have been plenty but this message comes up nearly every tile I check my mail.