Virtual PC is free, which is good. It is reasonably fast and functional, which is also good.

It has some bugs, which - surprisingly - isn't good. One of those is an apparently ubiquitous "Network adapter... failed to initialize because the address is a null address." A solution can be found via this blogpost. Weird, but works:

1. Examine the key value at HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Class\{4D36E972-E325-11CE-BFC1-08002bE10318}\<nnnn>\DriverDesc to locate the desired host adapter where <nnnn> is a four digit number.
2. Look at the GUID value for the NetCfgInstanceId key value.
3. Add the DWORD key at HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\VPCNetS2\Parameters\Adapters\<guid>\Flags and set the value to 0 where <guid> is the GUID found in step 2.
4. Restart the computer.