Master boot record
The term MBR is a bit of a misnomer since it includes four 16-byte partition records
and the code for the bios to load into ram to continue the boot process. This code is relocated to 0x7c00 iirc - it had to work in 64K pc's. The MBR always resides at the same place (head/track/sector) on all drives so the bios can find it.
A drive will 'boot' only if it has an MBR and one of the partition records in the MBR is marked as bootable (system) - which should only occur if it's been formated and contains a valid OS. A drive can have an MBR and still not be bootable. Or it can have no MBR.
This distinction is made since at one point, fdisk (the partitioner) could work successfully - update the partition record(s) - but not update the entire MBR if it already existed. Formating would also work correctly since the MBR only needs to be read. The Disk Management utility, found in Windows 2k and later, combines the functionality of fdisk and format. I personally don't know if it re-writes the entire, pre-existing MBR each time there is a modification or just the particular partition record. It seems like it should, to guard against malware, etc, but there have been compatibility issues with alternative OS's such as Linux, so it might not.
In the wikipedia article, the Structure of a Master Boot Record shows a Code Area [0 - 1BC). This is the area that virus's have been known to diddle in.
Originally posted by: dclive
Do this while you're behind a router, and don't go anywhere else (any other surfing) until you've done this and are fully patched, protected with AV, etc.
Succinct and excellent advice for everyone in all scenarios. The firewall in the router will protect from port scanners. There is no Windows firewall pre-XP/SP2 so even if SP2 is applied during the update process, there is a small window of opportunity for port scanners to do their evil deeds. This can be overcome by slipstreaming SP2 to an XP or XP/SP1 install cd - a fairly involved process.
There are a lot of opinions in the Security forum about malware scanners/protectors.