i agree!

i've always built my own computers from scratch, plugged in bits from older stuff with new stuff and it works like magic.
learning some PC hardware architecture and the way computers and their softwares work isn't that hard a task - when you know your way around a PC, you won't have to depend on some geezer at a computer shop to fix it, let alone wait for weeks on end for it to even reach the fixing desk in the first place.

MY TIPS:
make a list of the specifications your specific use requires. then check out motherboard companies and processor companies to find out which components suit you best. then check out your local computer parts dealer for prices and special offers and go buy the parts. buy the amount of memory needed/required for your specific use (usually 256MB or 512MB is enough even for image manipulation) and make sure it's the right type of memory. the faster (ie. 60ns access time) the memory, the more you have to pay but it's worth it.

so-you now have a motherboard, a processor and some memory. you should also have no problems building this inside your old PC case.
now you just need to consult your motherboards user manual to plug all the wires in, install your processor and memory modules etc. all the other parts you may have (hard disk, CD-RW/DVD drive, floppy disk, display card, etc) you usually just need to plug in. also remember that IF you have lots of components (a powerful processor, two-three hard drives, a CDRW-drive, a powerful display card), let alone components that require lots of power - remember to buy a powerful power supply too.
usually you'll get an ATX supply of 230 or 250W but if you have plenty of stuff inside your computer, buy at least a 300W supply.

when you've installed everything, fire up your computer and install the flavor of operating system of your choice. personally, I go for windows 2000 workstation because to me it's proven to be the most stable OS microsoft have made till date. XP looks like candy but is much heavier and if you're doing music with a computer, there's no sense in installing a good-looking lousy-working system.
then you need to set up the internet connection and download the NEWEST drivers from the manufacturers of your components. those that ship with the motherboard or video card, usually tend to be slightly out-of-date, so you're better off with some fresh drivers.

once you're through all this, and if you're still experiencing problems, try to find out what's causing it and what are the symptoms. ie. your computer is not firing up at all - possible solution is that some component is busted or installed the wrong way round. on the software side it can mean there's problems with drivers and you have to revert back to older drivers, find them through google or something.

..

well.. that's the most generic kind of help i can give. i don't know if it helps or if it's just bollocks but really, i don't suggest you buy a OEM (original equipment manufacturer) retail computer off the shelf (compaq/HP/dell etc) because their spare parts usually cost a whole lot more than spare parts for non-OEM computers.