Can't Rename .htm Files And Explorer Usage Suspicious

All the .html files on mywindows XP sp2 machine have been renamed to .htm and further when I try to rename them, the explorer CPU usage suddenly increases to maximum. But, some files gets renamed and others can't be renamed at all. And, for those which cannot be renamed, I get a pop-up error message saying "file is being used by another program and cannot be renamed.", and that's when explorer starts using all of CPU and system gets very slow.I have already done full system scan through Kaspersky and Malware Bytes Anti-Malware, but they have simple turned out to be futile.