I have a 1st generation Apple TV and a Canon printer that cannot be supported by my Windows 7 on my new computer. Can I download Windows Vista and have both on at the same time?
You can have them both installed at the same time, and reboot from one to the other.
You will need to install it into a separate partition on your disk.
If you don't currently have any unpartitioned space, you will need to shrink an existing partition by enough space to install Vista, at least 30GB would be wise. You can use a Disk Management in the control panel to to do this.
Once you have a free partition, you can boot the Windows Vista disk, and ask it to create a new partition in the empty space, then proceed to install into that space.
If by "on" you mean you want them both to be running concurrently, then one of them will need to be virtualised. Ideally, this is the one that isn't the one that needs to talk to specialised hardware.
Virtualbox is great for this, it costs nothing and is widely used so widely supported.
Yes and no
Microsoft provide Virtual PC which allows you to run Windows XP (I don't know about Vista) inside Windows 7
However since I believe it relies on emulating hardware for the "guest" operating system you would presumably still need Windows-7 drivers for the Apple TV and Printers. So perhaps it wouldn't help.
There are other virtualisation systems that allow you to run one operating system within another.
Here are some instructions for running 7 inside Vista using VirtualBox, this would allow you to use Windows-7 as a guest inside Vista with access to your printer and Apple TV.
Apparently, you can also run Vista inside 7 (by swapping 7 for Vista in the instructions) but then you wouldn't have drivers for your devices.
You might find attempting to get your old equipment working with virtualisation to be a complicated and frustrating experience. It may be a happier experience to just replace the equipment with recent versions that work with Windows 7.