What you need is to be able to boot up and then try and get back to before your brother downloaded whatever it was he downloaded, by using a previous restore point. Then run a bunch of malware scans to see what malware turns up. The restore points won't remove the malware, but if the problem was due to a software conflict, this will usually resolve the problem.
There are boot disks. There's one called Bart's PE. To make this you need your XP installation disk, or you can use one which is at the same level as your computer. So if the installation on your computer was from an XP SP2 home cd, then you need that. If it was an XP SP1 home, you need that. See if you can borrow one if you don't have one. Then you need to download the PE software at http://majorgeeks.com/Barts_PE_Builder_d4007.html
and see the instructions in the following tutorial:http://www.tweaksforgeeks.com/Setup_Barts_PE.html
The TweakGuide you mentioned is okay, no worries about downloading it, but it has Everything! so I think it might be overkill.See if you can get the comp to boot up.
Let me know how this goes?