Can’t open programs

Bought computer in Feb. Radus XL Core 2 Quad Intel Core 2 Quad Q6660. Had Vista Home Premium preinstalled. No Anti virus software. 15 y o son downloading to the max especially limewire. Now when you turn it on it usually says that it hasn’t shut down properly, I select to open windows normally, then it comes up with the vista welcome screen then the user account page – I select my user account and then the screen goes black with the following message:-
Windows cannot access the specified device path or file. You may not have the appropriate permissions to access them. Now I have a useless computer. Please help. Not a technical person.