Ever seen your desktop's performance grind to an impassable halt?
Or maybe the disconcerting sounds emanating from inside your tower have scared you into calling an IT nerd?
Before picking up the phone and racking up a ridiculously high phone bill; with the inevitable "Turn it off and on", flowing expertly from the customer service assistant's mouth without a hint of irony... You should open the tower and take your vacuum cleaner and suck all the dust out; needless to say turn the computer off and make sure you are doing this in a well ventalated room. If you're reading this, chances are there's more dust than in Tutankhamun's crypt.
Make sure to suck up as much dust as possible whilst not being too vigourous (remember: PCs are sensitive electronic equipment). Most of the dust build-up will be accumulated in the fan; try a small light bristled brush to dislodge any stubborn pieces of dust and maybe make sure you're doing it on a surface you don't mind being lightly (or heavily) caked in dust.
Once sufficiently cleaned, replace the cover of the tower and re-connect the power source. You should see a significant recovery in performance straight away and a drastic improvement to the noise, whilst operating.