How can I force my machine to use only the first processor?

Start a command session (Start, Run, cmd.exe).

Go to the root of the C drive.

Use the following command to modify the boot.ini file so it is not a system file, not hidden, and not read-only:

C:> attrib boot.ini -r -s -h

Open the file in Notepad:

C:> notepad boot.ini 

Locate the entry that starts the OS and add /numproc=1 to the end. For example,

multi(0)disk(0)rdisk(0)partition(1)WINNT="Microsoft Windows 2000Professional" /fastdetect /numproc=1

Save the change and exit Notepad.

Use the following command to reset the boot.ini attributes:

C:> attrib boot.ini +r +s +h