How often should i have my computer cleaned?

You don't need a professional.

The thermal paste should be fine for the life of the machine. If you're having heat problems you can replace it, but generally you never have to.
All you have to do is clean the dust out of your heatsinks and vents every 3-6 months or so, depending on your conditions. With an air compressor or a can of air (available for purchase at pretty much any electronics or office supply store) just blow air through the fan vents on the bottom of the machine and through the heatsinks on the side - you might be surprised at how much dust comes out. Just keep changing location and angle of your air stream until no more comes out.

hey thanks a lot for the easy and inexpensive cleaning tip!  
but i have no idea what the inside of a laptop looks like, are you sure that there are no wires or other types of things that can come loose if i blow air at that kind of pressure inside the vents?
thanks a million!

Don't use an air compressor to do that. Most of the dust there will stay inside. You should dismantle it , clean it with a brush, and change the thermal pasta if necessary.

if the op was going to take the computer in to be cleaned then he certainly doesn't need to be taking a laptop apart, a can of air will clean it sufficiently. And if properly applied thermal paste never needs to be reapplied (barring extreme circumstances), so seeing as the laptop was assembled by an OEM I'm gonna guess the thermal paste was put on properly and is just fine.

Having used an air compressor on notebooks and other heatsinks hundreds of times, I can pretty safely say that most of the dust does, in fact, come out. Also, the innards of the notebook are pretty tightened down, so you don't need to worry about moving cables and such.

A brush is woefully insufficient for cleaning out heatsinks, even if you take the notebook apart...you'll never get all of the dust in between the fins.

I use a vacuum (shop vac) on laptop fan screens and HS vents....been doing so since the early 1990's w/o issue.

http://www.homedepot.com/Tools-Hardware-Wet-Dry-Vacuums...

For desktops, I use this:

http://www.homedepot.com/h_d1/N-5yc1vZ1xgk/R-202018102/...

Been using this, or its predecessor, since early 1980's