How do you say my computer is fixed in Spanish?

As far as can read, the sentence "We fix computers for..." could be understood as they work for a company. Thus, a suitable translation into Spanish would be:

Nos dedicamos a la reparación de computadoras / ordenadores para (X compañía)

You could say, more literally: " arreglamos computadoras para (X compañía)" or "reparamos computadoras para (X compañía)".