Building a pc is easy. If you can fit a drive and graphics card, you can learn to build a pc from scratch.
You can get the parts in installments, but don’t leave the gaps between purchases too long.
Firstly, choose a cpu. A dual core intel isn’t too pricy.
http://www.komplett.ie/k/ki.aspx?sku=338082
that will dictate what type of mainboard you need (socket type)
http://www.komplett.ie/k/ki.aspx?sku=331973
then RAM. Don’t skimp here. Its not how much ram, its how fast the ram you have is.
Corsair twinx. Two, one gig sticks. Matched pair. See if you can find something nice here. You should get it for well under two hundred quid
http://www.ramboy.com.au/
hard drive. A western digital 320gig sata2 caviar drive, no questions ;)
case. Go antec, thermaltake, or lian li. They are quality, without gaudy. They will silence, cool and protect your parts, and provide easier assembly than a cheap crap case. Aluminum is good case material. Thermaltake seem best for price here, but look around. My antec is beautiful. Cost a lot, but soooo quiet.
http://www.pccasegear.com/category25_1.htm
you case may have a power supply. If not, get something nice. Your parts need an accurate and stable power source, without the power supply bleeding off too much as heat. Mine is a nice antec neo.
http://www.antec.com/us/pro_p_NeoHE.html
graphics card should be Nvidia for program stability. My latest is ATI, and I hate it.
A 8600gts gives great power for your graphics programs, and great for games.
http://www.guru3d.com/article/Videocards/426/
after all the big bits, its time for little bits. Two 120mm case fans. One in front to suck air in, and one in back to suck air out.
Thermal compound. It must be this arctic silver.
http://www.arcticsilver.com/arctic_silver_instructions.htm
of course your cpu should come with a heat sink, but if not, hunt down something good.
http://www.heatsink-guide.com/
as for assembly, there are plenty of websites with info. and lots of forums with people who specialise in hardware that will be glad to help you. good luck with this.