Getting a copy of XP for a new machine will likely be impossible at this time. Unless you purchased the copy of XP off the shelf as a retail edition of the OS (i.e., not included with any system,) it would likely be a violation of licensing terms to move that copy of the OS to a completely new machine. You could take up the matter with Microsoft; they might be accommodating in granting a transfer of license.
As for the days-long virus scan:
You need to create and mount (boot) a copy of Linux equipped with a scanner / malware removal tool on the CD/DVD drive and scan the Windows partition of the hard drive that way. Since the partition on the hard drive cannot infect the copy of Linux, both because it's on a non-writable media and on a foreign partition type, the scan will run at full speed and the malware can be eliminated in the time it would otherwise take Windows to do a full disc operation like a disk defrag, or a virus scan in which the system is not already infected. An iso to build such a scanner can actually be found at many of the leading Antivirus company's sites, or I'm sure there's tons of information out there on creating one from scratch. Pretty much anything that tells you it will create a boot CD to scan a Windows PC is going to be built on Linux.
Newegg is a great source for pre-configured DIY systems that you can build yourself, secure in the knowledge that someone who knows what they are doing has come up with the specs and parts list. I wouldn't necessarily rely on them for a truly high-end gaming system, but you're not looking for that, and I'm sure they are configuring very solid mid-range systems that are dripping with bang for the buck.