I am opening .html files in excel (they have the extension .xls, but contain HTML tables). Some of the values in these files are things like "1-5" which Excel is happily converting to "January 5th"

Because these are HTML files I cannot use the import data function to specify the column format as "text" and prevent this implicit conversion. Is there ANY way to make excel treat ALL the fields as text rather than happily assuming that my data values (which are actually ranges, eg 1cm-5cm but displayed as 1-5) are dates?

I've been an IT professional for 10+ years and this problem has me beating my head against the wall

anyone? Any help appreciated...

Try opening them in a text editor, and then saving them as text files. You can then use the Import wizard to format the data to your heart's content.