Hi,
I was wondering if it is totally safe to put a tower (upright) computer on its side?
Thanks.

Note that a tower is just a "desktop" PC turned on its side! The only reason you don't see desktop PCs much any more is they took up too much room on people's desks so they ended up on end, on the floor. Case makers just ran with that idea, and started making all cases, not just sever cases, towers.

Two precautions: (1) Make sure the tower lays on it's right side (looking from the front) so that the motherboard is at the bottom, and (2) Make sure that you are not blocking any ventilation holes. If your case has holes on that side, get some rubber foot pads to stick on there to lift it up an 1/8 inch or so to allow air to come under. The same precaution applies if you plan on sitting you monitor on the case - don't completely block air vents.