I have a spare harddrive sat inside my computer. It does have Windows XP on it but I have now got Windows 7 on my new hard drive and use that. Do I need a big power supply to run both hard drives, so I can use the old one for storage of movies. Does Windows 7 make it easy to determine which hard drive the computer uses?
You probably don't need a bigger power supply... but how are we going to know that if you don't tell us what you have already?
I'm not real sure what your asking by "Does Windows 7 make it easy to determine which hard drive the computer uses?" If your asking, does Windows 7 create a system partition and install itself there, yes but you will choose that partition during the install process.
Bigger power supply needed? Not very likely. Most recent hard drives use from 7 to 15 watts, so unless you PSU is already maxed out you are unlikely to have a problem. The worst-case possibility is that, with a loaded PSU, it is hard to start up everything all at the same time, so it is POSSIBLE to have a problem there, but still not likely.
How Win 7 would deal with two OS's on different drives depends on the history of how Win 7 was installed. Suppose at the time you installed it on the new drive you had already disconnected your old drive so that the Install routine had no way of knowing it even exists. In that case it would have made no record of a second OS. Now when you connect the old drive, Win 7 still will NOT go looking for any other windows lying around, and it will be happy to boot and run normally. After it is running you should be able to access and use the old drive in My Computer, but Win 7 still will make no attempt to use the old XP unless you specifically tell it to do so.
In that scenario, it actually would be possible for you to force a change, but not through Windows itself. In your BIOS Setup screens one of the things you set is the Boot Priority Sequence. It always should be set to use particular devices you want, and NO others. So when you re-connect the old drive, check there and be sure it still is set to try (for example) your optical drive first, then your new drive (with Win 7) second, and no other options. THIS is how you exercise control over which disk is used to boot. IF, as an alternative, you were to set this to use your OLD drive instead of the new one, it would boot up into the old Win XP, calling that old drive your C: drive and re-assigning a letter name to the new drive. But unless you really want to do that for some good reason, don't.
Now, back to history. Suppose, instead, that at the time you installed Win 7 to the new drive, you had the old drive with XP on it connected up. In that case, the Win 7 Install system should have detected the old Windows and asked you if you wanted to create a dual-boot option. Assuming yes, it would have created a system that offers you on EVERY boot-up a menu to choose which OS you want to boot and use. Of course, if you later disconnected the old drive I'm not sure exactly what the dual-boot menu system would do - probably just realize that no choice actually exists, and proceed directly into the only choice - the new drive with Win 7. So, if that is the history, when you re-connect the old drive it would then be possible for the dual-boot system to recognize that a second choice has been restored, and offer you the choice again. But still, with that system you actually make a choice at boot time, and there is no confusion or trouble because Windows (whichever) knows what you wanted to do.
Bottom line: you should be able to reconnect the old drive and use it with no trouble.