Several things could be the cuase of this.It's onre of those things where you have to try a process of elimination.
First try going into Event Viewer via Administrative Tools in Control Panel and under Applications you should see a red error icon under Type column which will refer to system hang or crash with the date/time it ocurred.If you click on this it may give more info.
Other things to consider are ; driver/ progamme conflicts (particularly are graphics and sound drivers up to date),temperature rise,failing or inadequate power supply, faulty memory.
Sorry can't be more specific but answers to this type of problem are sometimes difficult to pin point.
You might ask yourself if this problem only started after you added new hardware or software.
Hope this gives some pointers.