How about this Dell system unit for only �211
http://configure.euro.dell.com/dellstore/confi g.aspx?b=&c=uk&l=en&oc=D032002&s=bsd
Original price = �188 (plus VAT).
Replace Vista with XP, still �188
Reduce warrenty to 1 year = now �109 (plus VAT)
Change disk size to 160Gb = now �129 (plus VAT)
Go to basket and it comes to �211.57 (with VAT and delivery)
That is a good price for a Dual Core, Windows XP, 2Gb of memory, 160Gb hard drive +keyboard & mouse.
You can use your existing monitor and speakers, or buy new ones for around �100 (dont buy the Dell monitors they are overpriced).
That comes to a total of �300 which is �100 less than the repairers are saying it will cost.