1.5 firefox

I had the old version of firefox 1.5 which I liked. My sister came
over and desided my computer needed updating. My computer has not worked right since, my bookmarks are all gone I can't get into my e-mail. This computer is shared. I now have the Portable version loaded just so I can have something close to what I had before, but I have to do back flips to get into it. How can I get my old version back. And yes I have tried to do a computer restore for a back date but somehow it seems to have taken that abality away from me also. My computer has been possesed since the download version of the 2.0.