If WinXP is not loading properly (because of suspected virus/malware) is it possible to run
ClamWin from a bootable USB pen? If so, how do you make the pen bootable?

I believe you need an OS on the boot media--pen, Cd, or whatever. You can hit F12 at the proper time during bootup to switch to other boot media.

Just for starters, you might try booting your computer in Windows Safe Mode and run a scan. If the computer has been on, there's a chance that your ClamWin signatures are up-to-date because Clam uses https for updating.

You could try building the BART PE CD from another computer. I believe it includes ClamWin Portable. I've never had any luck building it, however.

If you don't mind using another AV besides ClamWin (God, forgive me!), get Dr. Web's Live Cd at https://www.freedrweb.com/livecd/ on the web. The page has operating instructions for you to follow.

Several other AV companies also have "rescue" CDs, including Kaspersky, Bitdefender, and F-Secure. I like Dr. Web's, however, because it's the smallest, it updates, and I have actually used it--it works. Here's a page providing information about them: https://blog.brothersoft.com/tag/xp-rescue-disk/ on the web.

Regards,