If the copies of XP and Office are original and you still have the disks,the monitor is an LCD and you have a decent PSU it should be worth at least between £150 to £200
IMHO. The 8800GT should be worth on its own between £40 to £50.
IMHO £100 is quite low and you have to only look at the prices of refurbished P4 PCs which have no Office,a smaller monitor,less RAM,less storage and a rubbish graphics card too.Also the single core 3.6ghz P4 was also almost the fastest one Intel did and is usually much faster than the one found in most refurbished PCs too from companies like BigPockets.