Computer Running - Failure To Initialize

i keep my computer running all the time. At irregular intervals approximately every 2-3 days the computer will partially freeze. I say partially, because programs already loaded continue to work, but any attempt to run any new application produces the message "failed to initialize". So for example I can still use or save an Excel worksheet that was already open but if Excel were not open and I try to open it I would get the "failed to initialize" message. Outwardly the computer appears fine - it is only when I go to use it or realise I haven't had an email in a long while that I suspect it has "gone again". Re-starting the computer without pulling the plug takes 30 minutes, but does eventually work after several non-responsive programs are terminated (not the same ones each time). All very frustrating....