without getting too involved, a second hdd with linux installed to it with lilo or grub installed to mbr, and a backup boot floppy made might work. copy the linux partition contents that you want to save to the second linux hdd install. disconnect the linux hdd, nuke, and format the winxp hdd, and install winxp. reconnect the linux hdd and boot up the linux partition on the second hdd with the boot floppy, and then you'll have to reconfigure where lilo/ grub is installed, and try and get it placed back to the first disks mbr.
a win98 start up boot disk can be used to wipe the mbr:
fdisk /mbr
winxp i believe has a fix mbr option when booting the install cd up, and choosing repair/ recovery option. this will wipe the mbr as well.
good luck with the above "maneuver". hopefully it will work
there is another option (probably tons of options, including forking over some money for packaged products at the comp store) using the copy command (cp) with the archive switch (-a). this will require partitioning, formatting, and mounting the second hdd, and then copying the contents of your linux partition to the second hdd. this will still require some work regarding editing your boot floppy to change the location of the linux partition to boot, and get boot loader back to mbr after winxp is installed, or edit the untouched bootloader in the mbr with the new linux partition to boot on the second hdd.
this can be quite frustrating to a newb- it was, and still is to myself. path of least resistance seems to be a reinstall. expert install is better so you're more in control as to what and where things will go. i started with redhat 7.3, and printed out the install guide to have handy during the install learning phase. this allowed for evaluation of options at each install step...
of course, you might end up reinstalling everything, so burning backup copies of all essential data will be an important step to not skip.
i would recommend a fat32 partition on the windows drive. this can be used to download iso, and backup files for both windows, and linux. this can be very handy to have. an ntfs (journalled) partiton for winxp, as well as a ntfs backup partition for xp can be handy as well.
in terminal type
man cp- look at similar terms listed.
also man mount
man cfdisk (log in as root, or su to use it)
cfdisk /dev/hdb sould be sufficient to get the second hdd loaded so you can create the partitions, types, and write to partition table.
man make, and
man fs should give some ideas on formatting linux partitions, and swap space.