Problem with reply and new emails on Outlook

I'm running a HP Compaq Desk Top with Windows 7 and use Outlook for my emails.

Some days (but not all) I can't get any response from my Reply button or New email button. The only way out of this impasse is to log out, then log in again.

It's not life-threatening, just B*** annoying!!

I've thought about more RAM but my genius son seems to think it's a software problem, not hardware.

I run Microsoft Security Essentials and scan my computer every Saturday night, so it doesn't appear to be a bug in the system. Plus the fact it's only happening intermittently, not every time.

Any ideas would be most welcome!