I was using BES (Battle Encoder Shirase) before to limit CPU usage per process, especially for OS Virtualization. Running three or four bots with a single core processor (mine is A64 3500+ 2.21ghz) could become really unstable with no limitation; and it allow me to use my host system while multi-botting but..for listening music, editing code, watching movie, that's it.
Usually i set my vm such as : 256ram & 15-20% CPU; and this is enough for Diablo botting.
Recently, i've found this usefull software, so i decided to make a post and share it !
Process Tamer is a tiny (140k) and super efficient utility for Microsoft Windows XP/2K/NT/Vista that runs in your system tray and constantly monitors the cpu usage of other processes. When it sees a process that is overloading your cpu, it reduces the priority of that process temporarily, until its cpu usage returns to a reasonable level.
ProcessTamerSetup.exe
Also i suggest you to check the official Process Tamer page for more detailed info : Click Me
