HIDS on Windows Servers

I'm looking for a descent HIDS (host intrustion detection system) to
run on Window's server boxes and possibly on XP Pro workstations.

So far I've seen Tripwire and it's pretty impressive, although
expensive.

I've also looked at Symantec's Critical System Protection. It's
extremely inexpensive, but I can't find a review of it at all.

I'm also looking at OSSEC - a freebie system.

I'd appreciate some feedback on any of these products as well as
recommendations on others if you've had success with them.

Thanks.