The VMs may get suspended if the sleep\standby option is set in windows VM.
To modify the power options in a Windows Vista/7 virtual machine:
1.Click Start > Control Panel > Power Options.
2.Click Change when the computer sleeps link.
3.Set the Put the computer to sleep option to Never.
To modify the power options in a Windows XP virtual machine:
1.Click Start > Control Panel > Performance and Maintenance > Power Options.
2.Change the System Standby option to Never.