This is some times trouble with space on windows c:\
1. Maybe you need to have much more space on your hard drive for windows virtual memory at least 2GB on c:\ partition my recommendation is 4GB
2. You have to much mail in your inbox 10 000 mails, if you not need them try to export some of them and save on D:\ disk
3.You have to much running process on your machine, who eat lots of processor time.
4. You have anti virus who is checking all mail, try to disable mail check when you download.