http://frozen-roms.net/
or
http://www.dgemu.com/
The former has some popup problems when it redirects you to the download link, and is mostly in german, but no viruses AFAIK.
The latter has no popups, but an annoying points system and requires becoming a forum member (which is free).
The point system severely gimps your ability to download NDS roms, though - most are bigger than the 12.5MB limit per day, and to get more you must either post a lot without spamming or pay.
Best NDS emulators as of now are No$GBA and DeSmuME.