https://en.wikipedia.org/wiki/Welchia
Welchia, also known as the “Nachi worm”, is a computer worm that exploits a vulnerability in the Microsoft remote procedure call (RPC) service similar to the Blaster worm. However, unlike Blaster, it first searches for and deletes Blaster if it exists, then tries to download and install security patches from Microsoft that would prevent further infection by Blaster, so it is classified as a helpful worm. Welchia was successful in deleting Blaster, but Microsoft claimed that it was not always successful in applying their security patch.[
A retrovirus does transcription in the opposite direction. It’s RNA genome is used as a template to create DNA which is then used to hijack the host cell and replicate the virus. It’s a bit like a cuckoo bird laying it’s egg in the host nest, only once that egg hatches it turns the other eggs into cuckoos as well.
Retroviruses were entirely theoretical prior to HIV, as RNA is famous for being unstable. Anyone who has worked in a genetics lab can tell you how big a PITA RNA can be, as even a heavy bump to the lab bench can ruin an experiment.