For the harddrive, use firewire, or SCSI. LVD SCSI will give you 12 meters (more than 36 feet) and HVD SCS will give you 25 meters (more than 75 feet). Not sure about firewire, but I'm sure you can look that one up. If you use EIDE, you will be stuck to just a few inches. I don't know how many, but Peter could well be correct wit 18".
Video, as IaPuP said. We have a 12-15' extension here at work. I'm sure they are commonly available for presentations and the like.
Power. Well are you talking before or after the PSU? If it is before, as long as you like, within reason. Go down to WallyMart and pick up an extension cord. It will have a chart of length, current, and wire guage. The longer it it the heavier a guage. Smaller numbers represent a heavier guage. You could easily go 100' with a heavy guage WallyMart extension. If you want even longer, you'll need something heavier that might involve a comercial duty entension. Go to Lowes, or Home Depot and you'll be able to get 250'. The heavier guage will minimise the power drop. And remember, chances are the computer you are reading this on has 100' or more of 12 or 14 guage wire between it and the service panel.
If however, you are talking after the powersupply, then you are more limited. I assume you are not as this would involve more work, but if you are I suggest you open up the PS and solder on heavier guage wires. You should be able to go 1 or 2 meters with only heavier wires. Beyond that you'd need even heavier guage wire, and perhaps some additional noise filtering circuitry cloe to the PC.
Keyboard and mouse should be able to be rather long. Again, for presentations people make extensions. If that is not long enough, either go wireless or USB with a USB hub in the middle to act as a repeater.
Audio, just keep stringing up the wire. Talk to RadioShack and they'll sell you a huge spool of heavy duty audio wire and tell you how many feet you can have. I looked into this once and 50' isn't a huge problem if you get shielded audio cable.