There were changes made in SP1, re searching all files rather than skipping files marked as binary files (though it defaults to skipping binary like dll etc).
As you're an ASP programmer you know more about the IIS search engine than us non IIS people. The IIS engine handles all search in XP (regardless if indexing is off or on). But the docs indicate that you can use any filter for any file type.
ASPs use the HTML filter as specified HKEY_CLASSES_ROOT\.asp\PersistentHandler (will be set to {eec97550-47a9-11cf-b952-00aa0051fe20} which says it's HTML filter at HKCR\CLSID\{eec97550-47a9-11cf-b952-00aa0051fe20}). If you use the plain text filter it will parse the file as unicode if it's first two bytes says I'm a unicode file or as ANSI if there is no unicode headers (unicode text files have a two byte binary header specifing the encoding). Therefore change the filter to {5e941d80-bf96-11cd-b579-08002b30bfeb} (text).
These are the filters
MIME Filter Multipurpose Internet Mail Extensions (MIME) (OE files) mimefilt.dll
HTML Filter HTML 3.0 or earlier nlhtml.dll
Microsoft Office Document Filter Microsoft Word, Excel, PowerPoint® offfilt.dll
Default or Plain Text Filter Plain text files - Default Filter query.dll
Binary or Null Filter Binary files - Null Filter (doesn't filter content only file properties) query.dll
Other manufacturers are supposed to supply their own (EG MS should have provided a filter for ASP with IIS, Adobe for Acrobat files, etc).
This engine is far more powerful. It allows google type results, abstracts, and has a full query language (though not on the content).
