make sure you have your virus protection software updated, then run a full system scan.
if that doesn't work, try looking on the microsoft site and seeing if you can download the latest version of Outlook Express, then re-install it.
past that, i'll need some more information:
first, is this only happening in Outlook?
second, will copy/paste work from the menu?
third, if it does, plug a different keyboard in and see if that works.
fourth, if it doesn't, have you spent four hours on hold with microsoft today?
i strongly recommend against the fourth option. mainly because MS tech support people are very mean and misunderstanding people. also, thier only answer is 'update/upgrade/reinstall...' the reinstall i don't recommend. one broken microsoft program isn't going to be missed. get Thunderbird from mozilla if you need a mail program.
sorry, enough ranting from me. basically, i'm struggling to keep myself from scolding you for using outlook express, knowing that it's a good program, that just seems to constantly break on me too...