Firefox should be in Add/Remove Programs in XP. If not, it may not have installed properly or the install may be corrupted. Follow the instructions in this KB article from Mozillazine to manually delete it.

Another method you could try is to do a full install, so that it does show up in Add/Remove Programs, and then uninstall it.

Hi amkeew, I´m not exactly sure in XP as I have Vista but you should be able to install the 3.5 ontop of the old FF. In FF just press Help-> Check for Updates then you should be asked if you want to update to 3.5. Matt

Thank you madmacs and LH. I will try to reinstall the old version and then uninstall it.