How can I stop my Macbook from going to sleep whilst I load the operating system (10.5) on?


How can I stop my Macbook from going to sleep whilst I load the operating system (10.5) on?
I'm trying very unsuccessfully to reinstall the system from scratch, so that I have a clean computer. I'm starting with 10.5 and then going to upgrade in increments from that to the last operating system the computer will run, which is 10.6.8.
Firstly I've wiped the hard disk in Disk Tools and then started it up from the master disk of 10.5, and its telling me it will take 64 hours and some minutes to install, so I can't keep moving the mouse to stop it going to sleep for that long.
I know it sounds as though something is wrong, but I have run both Disk Tools and Tech Tool Pro through it and both say there's nothing wrong with either the computer or the hard disk, and it was running fine (if slow - but that's its age!) prior to this clean install.
This computer is the first of the intel Macbooks and is a single core intel processor with 1gb of Ram.
What I need to do is to stop it going to sleep during the install, as every time it does it just halts the install and over night, for instance, I'm no further forward than I was the night before, even though I left it going all night.
All I'm trying to do is get it to 10.6.8 so that I can run Bootcamp on it and turn it into a PC to do my accounts (and no, the software we use only runs on PC, but Paralles, VMWare Fusion etc still make it painfully slow to use, and It's bad enough having to do accounts without having to wait for it to do every move...)
I can't go straight to 10.6 as the only copies we have of this are "drag and drop" upgrades that Apple supplied with 3 of our other Macs. Also the computer won't read dual layer DVDs (too old) so I can't load the full copy we have made of the OS.
It's simply called Photos and there is no options/setting thing on it. I have windows 8, it's a stand alone photo library viewer.
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