You get the headaches. If you installed the usual, then the stock setting allows outsiders to use the SMTP server to spam. If you can't disallow that (it takes time to digust and learn how to configure) start with the stock firewall and block incoming requests from outside your LAN.

I doubt the Linux machine has a boot virus, but you can google that answer so I'll not duplicate fine web content on that issue.

If this is some other XP machine, the boot is with the XP CD. You can rewrite the boot sector with the RECOVERY CONSOLE commands such as FIXBOOT and more.

Bob