Where I work I often have to develop (and bug fix) in an old system (.NET 1) whos code is complete spaghetti - with little thought given to variable names, program structure nor comments.

Because of this it takes me ages to understand what bits need changed, and I often 'break' the existing software because I've made a modification. I really really want to spend a couple of months (with colleagues) going through it to refactor but existing developers both can't see the need - nor think theres time for this (the system is massive).

I dread having to work on its code as it takes days to fix something only to find out I've broken something else. This obviously makes me look incompetent - so how can I deal with this?