|First check if you have too many email that you are saving and not deleting (outlook express has a limit, outlook and mozilla thunderbird allow more)|
Second, use your ISP's webmail site to see what is not coming in. You might find an unusually large file (review and delete there unless you need it) OR. You might find some junk mail that your virus scanner is hanging on analizing it so you get a time out. (If you don't need it, delete it)
Third, go to Start>Accessories>system tools>Disk Clean Up and select everything to clean out and compress.
If those do not help, post back system spec's, you may be short on RAM