Slow performance

Hi, at the moment my PC is running very very slowly, and I
can't understand why. I double click on a file and it
loads about 60 seconds later. However if I enter the
program and then open the document from there then it
loads much quicker. Whats going on?
Also when trying to shutdown the PC the computer won't
respond, then the dialog box appears saying that a program
may be busy or is waiting for a response from me. I click
on end task and then the PC finally shutdowns. Strangely
the program name doesn't appear in the dialog box.
I haven't loaded any new software in the last couple of
weeks and I've used msconfig to see which programs load at
startup and there aren't any new ones. I've also used
Ctrl+Alt+Delete to see what is currently running in the
background but again nothing new is running that could
cause a problem.