Compilation won't stay in Debug

I keep getting a weird error -
"the following module was built either with optimizations enabled or without
debug information..." (followed by path and more)

then, following the error, I find that the compile configuration is set to
Active(release)
(vs.net 2005)
I set it to debug - it goes right back, every time (even after removing all
bin and obj directories and removing all Temporary ASP.Net files)

any ideas?