I have a Gateway 500S, Pentium IV computer preloaded with XP Home Edition, SP3 (never had an installation disk.) I use antivir and zonealarm over the OEM antivirus/firewall, have updated spybot and adaware, checking from time to time. sometimes I forget to defrag, and have most of the automatic updates turned off because these slow me down (using dial-up) and something always seems to be updating in the background. Was downloading different metatrader4 platforms only to discover many contained trojans, antivir caught them. Still, system bogging down, a tech friend thought might have malware, found 5 administrators (I run without password) and deleted some. Now the system won't boot, gives the above prompt. Gateway suggested I secure an XP CD without reg code (these exist?) to repair the missing or corrupted files, restore administrator priviledges. Another friend lent me his XP corporate copy, tools load, but when I opt to repair, "A problem has been detected and Windows shut down to prevent damage to your computer. BAD_POOL_CALLER." Since windows won't boot, we can't determine what most recent software installations (or possible malware) might be causing this. I don't know DOS to use command line tools. One possible saving grace is I have Ubuntu on separate partition and same HD, but doesn't communicate with XP; the two OS are mutually exclusive. Too much critical data on this computer, a clean fdisk/reinstall is out of the question. Can I overwrite the corporate XP to my Home Edition and then change the key code back, without losing data files & apps? Can someone lead me through this?