When you need one is the best time to buy. Honestly, there's so little profit margin on computer systems, that prices are always generally the "best" prices.
Dell has a habit of running all kinds of crazy coupon deals practically every week, but most of them really aren't anything particularly noteworthy.
Basically, if you buy a system retail, at a place like Best Buy or Office Max, you're going to pay a little extra, and have them try to shove a bunch of worthless warranty programs on you. All those back to school promotions really just get rid of the retail markup as a ploy to get you into the store to buy other stuff.
My advice would be to build your own system. Do it right, and you'll have a rock solid system that you can upgrade in a very modular fashion that will save you huge amounts of money in the long term. However, if you're not interested in that, then don't wait for a particular time of the year or anything like that. Just buy one when you need it, and avoid those $500 or less systems unless you enjoy having your computer crashing due to poor quality hardware, among other problems associated with the corners cut to make that price point.
When is the best time to buy a computer? I've been under the impression that back-to-school time was the best, because that's when there are usually a bunch of sales and stuff, but is there anything else to consider?