Ideally you need to get hold of an XP CD and do that full format and reinstall. In absence of that however, you can try the following:
* Disable system restore
* Run
CCleaner
* Run
MalwareBytes full scan and delete any malware found. Reboot.
* Run
HitMan Pro and remove any malware found (needs free licence). Reboot.
* Run
SuperAntiSpyware and remove any malware found. Reboot.
Your machine should be clean, hopefully. But check with something more in-depth like GMER, Dr Web (scan takes a LONG time but it's good), or even a Kaspersky boot CD. Once you're pretty sure the malware is gone, install a decent and current multi-layered anti-malware suite (such as Comodo free Internet Security) and see how you get along. Comodo also has a module in Defense+ that allows you to see all running processes, and whether they are considered safe (this uses cloud scanning tech). You can terminate and block any untrusted processes that may still be lingering.
This isn't a comprehensive list, but it's something that's easier to "do" when you know how than to explain in basic terms on a forum post.
Since you have your own machine you may find it easier to remove the hdd from your sister's machine, and add it as a slave in your own machine. Boot from your own hdd into Windows, and scan her hdd from there initially. That has the advantage of not having the malware on her machine active (often meaning easier removal), but it's not essential as the above checklist will pretty much do the job anyway.
Good luck.