One thing to be aware of, is that three years after Vista and two years after Server 2008 came out, many system admin programs, including network monitors, don't work properly under the new security environment introduced by both OS's. This is known as the "Session 0 Problem" and it afflicts Windows programs that run as a service (many, many admin programs!)
I have to abandon a program I love for the poster's problem (and mine)--Servers Alive--since the current version has the "Session 0 Problem" and the developer doesn't know when the next version will be out.
My shop is a non-profit org for whom $1200, let alone the $10,000 for Orion, is very high for what it needs to do (essentially page me when something goes down.) Linux is the odd OS out for me.
If a vendor can't assure me that they've fixed the Session 0 Problem and made it fully compatible with Windows 2008 (R2), we're not going to put forth the money. That makes it especially hard to find a good program.