It's dead easy to build your own. The hardest thing is fitting heatsink to the CPU, nowadays much easier than my old Pentium III & Pentium 4 Prescott (needed a lot of pressure)
1) Take anti-static precautions (wrist strap to radiator or ground yourself by touching the PSU with the PSU connected to the mains but not switched on) Get all parts set on your desk. Open case, remove both side panels, and motherboard tray if removable.
2) Fit motherboard standoffs to the case, make sure where the motherboard holes are
3) Fit PSU.
4) Fit I/O back plate
5) Remove CPU pin protection, fit CPU to motherboard. Remove MSF plastic film over pad. Fit HSF to CPU and connect power cable up.
6) Fit memory
7) Fit motherboard into case and screw down (not too tight)
8) Fit videocard (plus power cable) and other PCI cards
9) Set master/slave jumpers for all drives, fit optical and hard drives, connect PATA & SATA and power cables
10) Wire up front IO panel (power switch, reset, LED, audio, USB, firewire etc)
11) Connect up ATX power, motherboard power and any case fans.
12) Connect monitor
13) Switch on, Set BIOS options, CD-ROM boot. Install Windows.