Wipe that drive clean. Not just reformat it, but blow out any partitions you have and recreate them new.
Your process should be...
#1. Save any data you absolutely have to retain off on a removable media that can be set RW so like a flash drive, portable HDD etc... Remove the media once you have the data you want backed up.
#2. Repartition the hard disk, and fully reformat. Don't save anything on the disk it is all suspect at this point. If you now have one partition, make it two, the idea here is to overwrite the MBR.
#3. Perform your Windows XP install, deleting the partitions again, and creating the partitions you actually want.
#4. Prior to connecting to the network, install drivers, and a recent, good anti virus and anti malware. Do NOT install your applications or data yet.
#5. If you can connect to the wired network, use wired first, and apply ALL avaialble updates to your OS and Anti Virus / Anti Malware. You never mention what AV / AM software you are using. I have used AVG with good results, have moved on to Norton Internet Security as my ISP provides it free, and my company VPN has a script that looks for Norton / Symantec, or McAfee products before it will let you in. (cuss, fuss, blah blah blah...). Symantec / Norton Anti Virus / Internet Security products have served me well for a very long time I must admit.
#6. Keep re-running the updaters until they report no more updates available.
#7. Install and update your applications. Same as OS and anti virus / anti malware, keep updating until you can't anymore...
#8. Perform system hardening tasks. (host based firewalls, applying security templates etc...)
#9. Turn OFF Autoplay / autorun.
#10. VERIFY you have turned off Autoplay / autorun.
#11. Using a known clean, sufficiently large and empty removable media, perform a FULL system backup of your freshly loaded system (just in case).
#12. Plug in the removable media, and once it gets a drive letter scan the living snot out of it and its contents.
#13. Assuming you survived Step #12 in okay shape, copy your date from the removable media to your local drive.
#14. Disconnect from Wired network, enable WiFi, verify connection speed.
#15. Re-establish network memberships such as Workgroup, or domain memberships if applicable.
#16. Re-establish any and all local accounts you want on the box.
#17. Enjoy and try to not get infected again...