Memory Error

Since my last instal of XP whenever i close Outlook Express i get this memory error
The instruction at "0x77f83905"referenced memory at "0x009a4e98". The memory could not be "written".
Click on OK to terminate the program

Does anyone know why this error is displayed, what this error symbolises and/or where i could find out what this error means?
Thankyou for any help provided.

I got this before, but I solved it after reformatting and reinstalling Windows. If you don't particularly want to do that (although it's good to do every so often), you could try running Memtest.

Thanks will try the mem test now