The PCI bus is made to run at 33Mhz and AGP at 66Mhz. The m/board creates this frequency by applying a division on the CPU speed. Hence when CPU is at 66Mhz bus (celerons) the m/board applies a 1/2 division to create 33Mhz and 1/1 for 66Mhz. When the CPU is at 100Mhz (anything else) the division is 1/3 to create 33mhz and 2/3 for 66Mhz. Therefore when you overclock your CPU bus to 83Mhz the division is still either 1/2 or 1/3 for PCI and 1/1 or 2/3 for AGP.
If my BIOS only allows me to set 1/2 PCI for 83 then the PCI bus will be at 41.5Mhz which is therefore overclocking everything that is on my PCI bus i.e. cards.
In the BE6-2 the 1/3 PCI option does not work (even though you can change) until around 92Mhz CPU bus.
This is exactly the reason defrag screwed up your windows.