If you have to continue to reset the time and date, the button battery on the motherboard will need to be replaced.
XP will run as long as the computer lasts. Win 98 still works fine as long as you can get drivers for the hardware.
If your main concern is anti-virus, there are plenty of free anti-virus programs just as good as Norton or McAfee and better than Microsoft AV.
The only software program you may want to consider is something like Acronis True Image to make a backup of your setup in case of hard drive failure so you have all XP updates. I use it every couple months so when the hard drive fails, I do not have to reinstall Win 8 and all software from scratch, which takes a day or more. Already have used it once.
Computer viruses are always developing via the internet. They were just as problematic in DOS (remember stoned virus ??) and it is not any software companies fault. I believe cars are recalled many times for things that should have been caught before production.
Good luck.