The steps we use to debug searchfilters with MOSS/ WSS are listed below:

1.     Disable filter host termination, add Assert to suspend filter host when it starts:

MOSS  [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Office Server\12.0\Search\Global\Gathering Manager]

WSS  [HKEY_LOCAL_MACHINE\SOFTWARE \Microsoft\Shared Tools\Web Server Extensions\12.0\Search\Global\Gathering Manager]

"DebugFilters"=dword:00000001

2.     Restart the OSearch service.

3.     MOSS:  Start a full/incremental crawl

WSS:  Touch the file of interest by opening it and modifying it slightly, and wait a few minutes.

4.     When you see the filter host assert, attach a debugger to mssdmn.exe.

5.     In the debugger, sxe ld <filter>.dll  (or you can do this via Image File Execution Options)

6.     Set break points in your IFilter dll.

7.     Set symbols for your IFilter.

8.     Wait for Break Point to hit.

9.     Start debugging.