is there any firewall in between solarwinds server and windows 2008 R2 server ?
Can you try SNMP walk from solarwinds server to windows 2008 r2 server?
Have you tried WMI rather than SNMP?
What is NPM showing when you try to discover the host? Does it show up as ICMP only?
A good way to test connectivity between your NPM and node is to use nmap. Run a UDP portscan from your NPM host (nmap -sU <ip of windows host>) and see if the host is listening for incoming connections on 161/udp. If so, you have a problem with the SNMP agent on the Windows box. If not, you've got a reachability problem that you'll need to track down.
If you need more detailed info on this test, let me know.