I'd suggest you stop using XP for anything but the specific task you need it for and unplug the network cable except when you absolutely must take it on-line. Very definitely no browsing. Also very definitely no running as admin user.
Sooner or later there will be no anti-virus software available for XP. What are you going to do when the XP computer dies? Maybe you need to investigate virtual machines, noting an infected system running in a virtual machine can infect/attack the host system.