Sounds to me like a CPU load issue as stated. If you're running XP and still talking about your computer store, it's probably a fairly new PC so this shouldn't happen. Most new PCs will come with onboard sound which is mostly software driven - this means it relies on the CPU and the memory subsystem to chug out sound when needed.
If you're running a VIA board, definitely grab the VIA 4 in 1 patch - you can get it from www.via.com.tw
. Or look at your motherboard manual and go to their website. Usually you go to the support section and then software/drivers.
The PC pit stop site mentioned above is a fantastic one. You might have a heap of spyware running in the background chewing up resources. In XP, you can right click on the start bar and go to task manager to see what resources your PC and the programs on it are taking up. If you have virtually no resources, then PC pit stop might help you get rid of some programs that are taking it up...
If it's a hardware problem, I'd be thinking bad RAM - as most 'corner store' computer shops throw in the cheapest components they can find as the retail PC industry is cut throat. In my experience, the fmost dodgy cheap component is RAM (or maybe power supplies - but with more reason). If it remains a problem after patching, I'd recommend getting your computer store to fix it if its under warranty. They don't take much of a financial hit if its faulty hardware, and the local PC stores here in penrith are usually quick to replace faulty items. Take them to the dept. of fair trading if they won't fix it under warranty.
If its not a new PC, then it might be time to upgrade (egad, I hate that word - it always costs me money!). Or perhaps have a look around for a new sound card (a dedicated soundcard won't take up anywhere near as many CPU cycles). Check out http://www.computermarket.com.au
If you have any questions, PM me - hard to troubleshoot through email, but if I know what hardware you have I can point you in the direction of all the right patches!