Sywindt
If you have a multicore machine, this is easily fixed by setting the different clients to use different cores. I use a program called WinAFC (available both in 32 and 64 bit versions) to do this for me automatically.
In Windows 7, the "start" command allows you to set processor affinity. I don't have a Windows 7 box handy, but the syntax in Server 2008 is:
start C:\whatever.exe /affinity 1
That will use the first CPU/core (which, confusingly, is listed in Task Manager as 0). I don't use this, but it may also be possible to specify multiple CPUs/cores.
Windows XP doesn't have the affinity option in its "start" command, but a company named SysInternals that Microsoft purchased a number of years ago has a toolkit free for downloading called PsExec, providing another method to acheive this:
http://technet.microsoft.com/en-us/sysinternals/bb897553
psexec -a 1 c:\whatever.exe
That will also use the first CPU/core to run the program.
It will ask you to accept a EULA when you run it. This should happen only once, but in case it doesn't, bump this thread and I'll see it when I get home from work so I can remember to post the extra option to turn off the EULA notice. Can't seem to find it at the moment.