Applications Won't Load

On some nights my computer works perfectly fine, I can surf the net without incident, open applications, play games etc and everything appears to be fine.Then there are the other nights (which are increasing in frequency) where the computer boots up fine but then I run into application after application which refuses to load - I will click on the application and the "loading" icon will appear on screen and then just stay there.The only way out is to cntrl-alt-delete and bring up task manager, but this can also then refuse to either load or - if it does - will not respond when I click on the first offending application in an attempt to close that down.I also noticed on the last few nights that when trying to start a particular game clicking on the icon would appear to have activated nothing - yet when I attempted to start it again I would get a message saying that I couldn't start the game until the one I had running had been shut down - but nothing appeared to be running.My computer was built in January of this year. I have regularly run disk cleanups etc and run virus checks at least once a week and nothing shows up.