Question : With a wireless home network connecting to a third computer seems to slow internet connection speed to almost nothing what can i do to fix this

I have a wireless network of three computers one is wired to a Belkin wireless router which is connected to a D-Link 300g+ ADSL modem with British Telecom 512k service broadband. The other two are in other rooms with Belkin USB wireless network adapters. When using the internet on the wired computer and one of the wireless computers in the other room the speeds are very fast. However when connecting using the third the speed drops to very very slow speeds.
   Is there a way around this problem, i.e. could I wire one more of the computers and have the other wireless.
Any advice would be greatly appreciated

PC Specifications:

2000ghz +
512mb ram +
Windows XP

n.b. PC’s are within 20ft of each other so signal is no problem

Answer : With a wireless home network connecting to a third computer seems to slow internet connection speed to almost nothing what can i do to fix this

If I understand your comments above you get a slow down whether your 3rd machine is on wireless OR a cable but you only notice this when you do a download - this can be quite normal if you cannot allocate or reserve bandwidth for each machine. The one doing the download will tend to hog the Internet connection receiving multiple large packets and the other traffic will suffer as a result. In this event it is possible to timeout on DNS lookups etc even with only a second machine running.
However, if you really only get problems when you enable the 3rd machine on wireless then I would check for dropped packets etc due to interference affecting the wireless signal. Check for portable phones, baby alarms etc etc. Depending on the design of the router and the CPU speed it may not be capable of keeping things running well if problems occur.