Removing obsolete addons

I'm upgrading to 1.5 and have taken the opportunity to remove the files for some unsupported / unused add ons. To my surprise, even though I've started with a clean set of 1.5 files only (i.e. I completely removed all old files) I'm finding that old add ons are causing me errors. For example on the select payment method page I'm getting ' WARNING: Could not locate language file:' for an add-on I thought I'd removed.
Obviously, there's something lurking in the database I upgraded. How can safely remove these add ons. I've spent ages browsing the forum without much success.
Thanks for any assistance.

A comparison of files will quickly reveal what doesn't belong: http://www.zen-cart.com/wiki/index.php/Troubleshoot_-_Diagnosing_Obscure_Issues

And, if shipping/payment modules have been cached, then visiting the admin page Admin->Modules->Payment or ->Shipping will reset that cache, thus removing the database from the equation.

:clap:Sorted - many thanks!
Your post pointed me in the right direction - it was order totals that was the problem. Just went in there and it all went away.