Is all I want to do is convert or append to beginning of a the filename the last modified date/time, in YYYY-MM-DD_hh-mm-ss.ddd (or variations thereof; I'd like customisability). If anyone can find a utlity to do this, I'd be grateful, as I want all my digital photos chronologically organised.

I'd prefer any of the following formats:
2000-06-14_23-59-59.999.extension
20000614_235959.999.extension
20000614235959.999.extension
20000614235959999.extension
2000-06-14_23-59-59#001.extension

As you can see, it's based off either the modified date/time, in year-month-day hour-minute-second.decimal format or without the decimal, substituting with the photo number at the current second. I'd prefer a program where I can customise the format, so I can choose whatever format I want.

Thanks very much to everyone who replies...