By default, many linux distributions install their interrupt timer to 1000 Hz or 1000 per second.  This is great for systems that are routing VoIP packets or any system with a lot of processes or threads that need to get equal attention.  In fact, I used to recompile the kernel to increase the default from 100Hz or 250Hz for systems that had many processes.  This also works well for the host OS if you are running the freeware VMWare server.

A guest OS running inside as a virtual machine, however, may not be optimal if run at this rate.  Imagine running 10 virtual linux machines inside of a host.  This is creating the conceptual equivalent of 10,000 interrupts per second of demand on the host OS.  In most cases, if you are running a VM that requires that sort of attention, such as a Asterisk or Freeswitch system processing lots of calls, it probably not a good idea to run that in a VM.  Give it a dedicated box and all the interrupts it can get.

However, if you are running some lightly tasked or surge tasked guests, then reducing this 'tick' rate can really reduce the CPU load on the host and in many cases improve responsiveness. 

kernel /vmlinuz-2.6.18-164.6.1.el5 ro root=/dev/VolGroup00/LogVol00 rhgb quiet divider=4

In /etc/grub.conf or your equivelent, the keyword is to add the 'divider' option.  In this case, 1000/4 = 250hz.  Which is probably adequate for anything not requiring 'real time' such as a generic web application. 

Conceptually, if you think about the CPU cores having to divide up tasks, this also lets them work longer on each 'tick'. For applications such as a large database query, this can reduce query time a bit, ( ymmv ).

I divide by 4,5, or 10 depending on what the guest application purpose is.  Regarding windows hosts, I believe it is configurable via registry key, but I think the default is 250Hz, but I can't say for sure in the latest OS releases ( 7 / 2008 ).  There is a registry trick to reduce disk swapping in server 2008 ( great for dedicated, not so great for VM ) which makes a big difference and I will post that in another note.