Outlook express memory error msg?

I have a user in a branch office who emailed me that she is getting the following error in Outlook Express:
There was an error opening this message.
There is not enough memory.

This message appears when she tries to create a new email message. The machine has 96 Meg of RAM, 300+ MB free on the hard drive. Running Win98, and IE 5.5. We tried re-installing IE, and that seemed to fix it, but the next day she told me it was happening again. If she reboots, she can usually get it to let her send one or two emails, but not always.
She is not getting other memory errors in other programs.
Any advice would be greatly appreciated.