If you can open this box <i>without voiding a warranty</i>, do so, and see what there is to see. The "fizz" sound is probably something arcing, and the burning smell is probably the arcing part taking some damage. It usually means there's a short somewhere in the system.
which bit Fizzes (wow you sound like one of my customers - "my computers popping") the tower or the monitor - If its the tower - see if the mobo is shorting on the case. This can happen a lot in OEM systems - mass production generates mass mistakes. make sure none of your cards arn't touching any metal. Though if they are you'll be lucky if they are still alive. Like Kelledin said - If you can open the case with out voiding the warrenty do so. You don't want a house fire. If you are allowed reseat the components - If you arn't allowed then ringing an advisor at your hardware vendor should cover this on your warrenty - Ie advisor on phone while taking machine to bits.
<font color=green><b>AMD</b></font color=green> 'cos my computers worth it!