and I'm sorry I {almost} shouted that, but those pesky registry utilities are a pet hate of us Mods here, 
Bob's advice is great. We had a discussion just last week about a similar problem, and after trying various options the user noted that he was using Spybot Search & Destroy with it's TeaTimer background scanner running. He removed that and suddenly his problems were solved. It is sad, but this once great utility Spybot doesn't work well with today's anti-virus scanners.
I have a setup like yours. My main is newer, but I also have an XP system that is some 10 years old now and while it is still performing well, it does now need care and attention.
Since your XP computer is now used just for backup, (and perhaps other non-critical uses), may be it is time to pare it down to basics. Here are some options;
1] If Startup is slow, head over to MSCONFIG, (Start > Run, type in msconfig and click OK), and the Startup tab, and reduce all 'load at startup' software listed, except for the necessary ones. Necessary ones include all Microsoft, any firewall and anti-virus.
2] If you have Microsoft Office's "Quick Launch" toolbar installed, uninstall that. It loads at startup and that takes time.
3] Notification area icons. This is the area at the right end of the Taskbar, where the clock is. Just check that you don't have any unwanted ones now. There may be hidden ones and you can check those by right clicking the Taskbar, select Properties, and find where it is showing or hiding Notification area icons. Any you don't need now, find out how to shut them down.
4] No Desktop background, or screen savers. A minor consideration, but if the Desktop background image is large or is a BitMap, (.bmp), image, it can take a little time to render at Startup.
5] As few shortcut icons on the Desktop as possible. Again a minor issue, but if the Desktop has many icons, it takes time for the OS to check the software is still all there and then render the icons to display them.
6] Running Processes. You can check these in the Task Manager, (right click the Taskbar, select Task Manager, then the Processes tab). This is not an easy item as you would have to find out for yourself what each process is, and whether it can be removed or not. Google will help. Processes are often associated with "Services", so disabling those Services that are no longer needed could help improve performance. You can review Services from Start > Run, type in services.msc and click OK. The Elder Geek will help here.
7] You don't say what RAM this system has but always a good bet is to increase it to the max available. Possibly not a solution you would want for a system you don't use much now, so that is why looking at Processes might help as these reside in memory and can slow the system down.
Only run one anti-virus scanner in the background. Also, no need for any anti-spyware or anti-malware scanners to run in the background.
That's a start and I hope it helps.
Mark