If the program resists loading on the existing computer it's more likely that it's intended to run on a later (or earlier) operating system than what your friend has. The critical thing you need to know is what OS is needed.

Nearly any "low cost" Windows computer you will buy easily now will come with Windows 7 preinstalled, but some sellers may still offer ones with Vista. Despite what Microsoft wants you to believe it is still possible, with some extra searching, to find new or refurbished computers with WinXP, but you need to find out what operating system you need to run the program that's required. You may have a choice, with some programs, but cross-compatibility is a lot less realistic than is claimed by some.

Fewer mainstream makers are offering really cheap desktop machines now than a year or so ago, with the advertised prices from the ones who flood me with email "buy now or die" advertisements starting generally around $700 - $800 (US). You'll need to look closely at what you're getting to be sure of meeting your specific requirements, but offers of "free upgrades" for RAM and HD size are often available even for the minimally priced ones.

Most of the mainstream sellers also offer refurbished machines that may be at significantly lower prices, if you can find what you want on their web sites.

We've had no problems with a $500 desktop (Vista) purchased from a "local builder" about a year ago, but that builder actually is a member of a national chain (Ribbit) with some reputation. Curiously, their stores here don't accept credit cards or personal checks, so we had to run to the bank for cash; but that was (for us at the time) a minor inconvenience. Truly "local" shops that offer to build you one can quite easily build the equal of what the major makers offer, even if they do store their spare parts under the kitchen sink; but they can also build junk and it's often not easy to find out which reputation they've established - and whether they'll still be around when you need support.

The first and primary problem is to identify and what OS versions the required program is intended to run on.

John