Is it OK to vacuum clean the interior of the system unit?

Its also likely to cause more issues in the winter when the air is drier. I wouldn't recommend it if your noticing static when you touch metal objects during the day.

Most of the damaging static is untestable, and certainly not visible, before it causes the costly damage. There are instruments that will easily detect it.

But why take the risk when canned air and a damp cloth will do everything you need.

I second raybay.

It might be safe to vacuum once, maybe twice.

But there is still that chance for your vacuum job to fry the components in a PC, even if its 1/1000 chance.

There was a post on Techspot before about how to vacuum your PC safely. It had a full explanation on how static builds up, how it discharges, how it destroys components, and of course, how to overcome static damage.

From what I remember, the easiest way is to make sure either both vacuum and PC are grounded, or both aren't. Most vacuum cleaners are grounded, and most PCs are grounded too. Most damage occurs probably from people putting their PCs on carpets (ungrounded) and then vacuuming it (grounded).

So, just leave the power cord plugged in, and vacuum it, so both are grounded. Just make sure that your PC is switched off.

The grounding doesn't necessarily help. The static electricity is created by the swirling movement of the dust and hair itself. You will have the disasters only occasionally as it only occurs when the temperature, humidity are just right, combined with the moisture content of the dust, lint, and hair.
You can run your own tests using a spare computer and static meters. It is not difficult to do this. You can easily see the static charge build up and discharge when you use simple equipment.
Anyone who uses a vacuum cleaner puts their computer components at risk, as this vacuum cleaner induced static can easily destroy a motherboard or a power supply... memory, video cards... anything with a chip...
When the tests are so easy to do, and the problem is so obvious, it is amazing to me that experienced techs believe this is baloney.

Risk vs reward. A vacuum cleaner is the easiest way to clean the insides of a comp, and the cheapest. Given those simple safeguards (and maybe even without), most techs don't even run into static problems.

Of course, if someone worked in a busy computer store, where they probably service 500 computers a month, they'd see a few comps burnt from vacuum cleaner incidents, just because of the numbers. Its all in statistics, given enough tries, even with an infinitely small probability, it will still happen.