ResolutionIf you just upgraded to Parallels Desktop version 4, and now virtual machine (VM) pauses itself after a few minutes it means, that you have "System Standby" power-saving regime enabled.
Since Parallels Desktop version 4 the VM's hardware is fully ACPI-compatible. If after some period of inactivity Windows decides to go to sleep mode - the VM will be paused. Thus, the behavior is controlled by your Windows' power scheme. The idea behind this is battery power saving. The VM will pause itself after a short period of inactivity for battery power saving.
You can control this period in Start->Settings->Control Panel->Power Options. If you do not want windows to pause anymore, just set "System Standby" to "Never"