[Nyororin]

腐女子 is used almost exclusively to refer to girls who like BL. I think it would be very weird to translate it literally as "decaying female", even if that is what the characters themselves mean. It`s really a play on 婦女子, turning it into something negative.

As far as a non-derogatory way of mentioning your interest in "nerdy" things... I don`t really think there is a simple and easy way of doing it. Usually people just don`t go around advertising these things. Anime and manga are genres, not really looked at as specifically as they seem to be in English. Like, for example, you wouldn`t say "I like magazines" or "I like television programs". You would be more specific, as clearly you wouldn`t like ALL magazines just because they`re magazines, you wouldn`t like ALL television programs just because they`re television programs - the same goes for anime and manga. People usually don`t like anime just because it`s anime, nor do they like manga just because it`s manga.
This is why people who say they don`t like anime or manga (a normal answer from the average Japanese person) or who show a bit of hesitation about someone saying they like anime and manga may actually have a show or series that they adore. It`s not because it`s "anime" or "manga" - it`s a specific book/program.
Just think how odd it would be to have someone come up and tell you "I love magazines! Magazines are great!" Without any specifics, they would come across as flat out weird. But you probably wouldn`t think twice about someone saying "I enjoy reading fashion magazines" or "I always buy ***** when it comes out."

As for the computer/technology side - it doesn`t seem to be too common for girls to be really into that sort of thing. I have always found 元々ＩＴ系ですから･･･ to be the best way to explain the situation when I know how to fix friends computers or operate their brand new phones, etc. Or even こういうもの結構好きだから when it comes up. It doesn`t give any "obsessive" vibe, but makes it clear my interests.