The SQL Server 2008 ("Katmai") CTP 4 VHD that was published to MSDN Subscriber Downloads last week is made available as a 4-part self-extracting RAR archive. 

It can seem to have an issue in while self-extracting.  As shown in the screen cap below is it looking for en_sql_server_2008_ctp_4_vhd_part_1_of_5_.rar: 

This is obviously wrong because a) part 1 is not a rar - it is in the exe, and b) the download is in 4 parts not 5.  So if you see this prompt, browse for en_sql_server_2008_ctp_4_vhd_part_2_of_4_.rar.

Likewise when asked for en_sql_server_2008_ctp_4_vhd_part_2_of_5_.rar browse for en_sql_server_2008_ctp_4_vhd_part_3_of_4_.rar. 

Finally when prompted for en_sql_server_2008_ctp_4_vhd_part_3_of_5_.rar browse for en_sql_server_2008_ctp_4_vhd_part_4_of_4_.rar.  You will not be prompted for a part 5.

Once you have done that, fire up VPC or Virtual Server 2005 and enjoy!

Also, don't forget:  You can check out the LiveMeeting events on the new features in SQL Server 2008 available through the Connect site.

Listening to: Aphex Twin, Selected Ambient Works, Volume II