You only really have three options here, and the choice boils down to how much you value your time and money.
Method 1: The Easy Way Out
Travel as far as necessary to find a real locksmith, and pay them to take care of it.
(Alternatively, mail the entire computer to the locksmith, and hope it gets there and back in one piece.)
Bonus: the locksmith can also make you a new key, so that you can reuse the cable lock.
Method 2: Pick the Lock
Due to their construction, tubular locks are quite well protected against any improvised method of lock picking. For example: even if you rigged up some method of picking an 8-pin tubular lock one pin at a time by hand (using a rolled strip of metal cut from a soda can, perhaps), you'd quickly find that the pins spring back with more force than can be counteracted by friction alone; you'd need to actively hold all 8 pins in place at the same time in order to get it to turn. Additionally, since any of the pins can just as easily pop into their neighbor's slot, you may end up having to solve the entire lock all over again another 1-3 times (depending on how far it must be rotated before it releases).
Long story short: some models may have specific weaknesses that can be exploited, but in general, you'll need to buy yourself a purpose-made lock pick. After you have a lock pick on hand, the process is pretty straightforward!
Bonus: with the use of a lock pick, measurements of each pin can easily be taken. This information can then be used to create a permanent key, and the cable lock can be reused.
Method 3: Destroy the Lock
Depending on the make/model of the cable lock and the make/model of the computer in question, this method may or may not be practical for your application. The gist of it is to cut apart the body of the lock, exposing the two-piece core: these are the two-peg outer core and the T-shaped inner core that rotate relative to each other to engage/disengage the lock. Once the outer lock mechanism is completely cut away, you should be able to rotate these two pieces by hand, releasing the computer from its grip.
Cost: low (assuming you already have the necessary supplies).
Tools needed (alternatives given where appropriate):
- Rotary tool with diamond-grit cut-off wheel to cut the lock open (some lock components will be hardened tool steel; you can try using other cutting tools like a hole saw or jumbo bolt cutter, but they may not, you know, cut it.)
- Painter's tape to seal all computer openings; metallic dust will fry a computer very quickly if you let it. (packing tape, duct tape, etc. also work, but they tend to leave behind a layer of sticky residue)
- Foil tape to protect immediate surrounding surface of computer from sparks (paper tapes can burn, and plastic tapes can melt. regular aluminum foil works, however; just make sure to secure it well enough.)
- Big ol' pair of pliers (pipe wrench, vice grips, what-have-you) to hold the lock steady without putting your hands in harm's way
- Good solid pair of safety glasses--or even better, a full face shield (your eyeballs don't like flying specks of red-hot steel any more than your computer does)
- (optional) Good pair of work gloves (must be heat-resistant; rubber and artificial fabrics are prone to melting to your skin, which would be worse than not wearing any gloves at all)
- Shop vacuum and/or damp rag/paper towel to clean up metallic dust before removing tape
Notes about technique:
- Back up the computer's contents first. This will probably void your warranty.
- If teardown instructions are available for the computer, look them through; if it's easy enough to separate the sensitive electronics from the portion of the housing with the attached cable lock, it may be worth doing so.
- Be thorough about protecting the computer with the tape. Even the tiniest of openings could let in a computer-killing flake of metallic dust!
- Clear a good-sized workspace on a flame-resistant surface (no veneers or other "pretty" surfaces, although a solid wood work bench would be fine) with plenty of ventilation.
- Take it easy. Cutting hardened steel too aggressively can result in damage to your cutting tool, the computer, and yourself if you're not careful.
- Position the cutting wheel such that sparks are directed away from the computer (and away from yourself, for that matter). The foil will help, but not that much!
- Take it in stages. If the lock heats up too much from the friction of the cutting wheel, it could damage the computer.