I've search my second blog for a trick I did a long time a go: popup / drilltrough for RS2000, well Id didn't find it so here's the code:
in the navigate URL [ AKA "action" ] you put an ASP/APSX page
which has the following
this is a sample which has worked for me as a walkaround from the RS2000
in the MyNewPopup.aspx i've used an IFrame to show the subreport.
for a simpler option in SSRS2005 here's How to create a hyperlink in Reporting Services that opens in a new window