I'll probably try to get screenshots later, if needed though i think i can describe the issue with sufficient clarity to replicate in words. My dad used to use an older version (3.36 i think) of paint.net and pasted from a site that uses unicode to render tamil - This is an example of a site he uses, and should render correctly on current versions of firefox -I'm using 3.5.5
Copying and pasting a passage from there, renders properly in paint.net 3.36, Word 2007 and firefox. It doesn't in notepad, the latest version of paint.net (3.5 something i think) and paint.I'd suggest using arial unicode to test, since its a common font that seems to work correctly with the script, in cases where it works. I'm supposing there's a change between the two versions, and thats what's causing the issue. While its a single, not too common language that i don't think the developers speak, Its still a rather curious bug, and i figured i'd bring it to your attention
EDIT: The system is running windows XP SP3 as well. I guess i should have mentioned that