If your pc is connected directly to he wall socket and not via a surge suppressor, switching off at the wall socket could "spike" your pc and cause further and possibly expensive problems.
Will it not shut down by depressing the power switch for a minimum of 4 seconds?
There are a number of things that could cause symptoms you have from a failing power supply to a corruption in Windows.
Did these symptoms appear after you upgraded or added any additional hardware or software?
Have you tried pressing F8 on boot up and selecting Start from Last Good Configuration (or similar words)?