The application server, agent, servicepages server, WebConsole server, and Client 2008 use Sun (Oracle) Java.
Only the "old" / "legacy" client uses MS J++ (MSJVM), which doesn't support on Vista or Windows 7 anymore. But the MSJVM can still be installed on XP, 2003, 2005, 2008
For the application server, Windows 2008 32-bit is supported since sp35 (see ITSM009828 in the release notes), and 64-bit is supported since sp38 (ITSM009855). Note that the application server is still a 32-bit java program, so use the 32-bit release of JRE/JDK 6.0, even on 64-bit OS.
The above applies to Windows 2008 "R1", but there is also Windows 2008 R2 which comes as 64-bit only.
Although not officially supported, there are customers who managed to install and run the 32-bit app-server with 32-bit Java 6 on Windows 2008 R2 successfully.
Which Client 2008 features are lacking for you? The Differences Guide explains about differences in layout, forms, views, but those should not be serious. The single biggest advantage of Client 2008 is that it works on Windows 7, whereas the legacy client doesn't.
HPE Software Support
If you find that this or any post resolves your issue, please mark it as an accepted solution. Kudos are welcome.