I recently purchased an old house and the toilet drains slow and bubbles. I cannot locate any plumbing venting at all. The plumbing is all located within 4 ft of the main drain going out through the basement wall, with the kitchen sink, bath sink, tub all connecting to the toilet line then out through the main line. The main line is in the basement wall directly below a window, is it possible to run a vent off the top of the main connection out the window and up the side of the house? Will this help relieve a bubbling toilet, barring any clogs of course.

A bubbling toilet indicates back pressure caused by a partial blockage. Snake the drain downstream from the toilet and see if that doesn't help.
A good way to tell if you're vented is to look under your fixtures at the traps. If they go into the wall then chances are you're vented, however, if they go into the floor it's a "S" trap and your house is indeed unvented.

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The main line is in the basement wall directly below a window, is it possible to run a vent off the top of the main connection out the window and up the side of the house? Will this help relieve a bubbling toilet, barring any clogs of course.

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This will not vent your house. Code mandates that every fixture that has a trap must also have its own vent. Good luck, Toim

If all of the fixture drains come to the main drain line and I vent this drain will this qualify as a main stack vent. Is it OK to run this dry vent at 90 degree angles out the window and above the roof line of the house?

As I've stated in a earlier post this will not vent your drainage system. If you install a vent off the sewer cleanout you will have accomplished nothing. Your fixtures will still all have "S" traps that were outlawed years ago in all the states and Canada. If your aren't going to repipe the drainage to accept a vent on every fixture that has a trap then save your time and money and keep the present set up. Regards, Tom