installing Ubuntu from a flashdrive alo: ngside windows
 

This is my first day trying Linux. I can use Ubuntu from my flash drive, but I want to install it in a partition alongside windows. When I try to do this, I come to an 'allocate drive space' window, but whatever I do I get the error message: 'No root file system is defined. Please correct this from the partitioning menu.' I just don't know what this means, or what to do next. I'm loathe to ditch windows, and I don't want to have to use a flashdrive all the time.

You don't indicate whether you are using Ubuntu 11.04 or an earlier version? In 10.10, you select a partition in the main window of Allocate Drive Space, then click on the Change tab at the center and a new window opens where you can edit a partition. You can set the partition size, filesystem type and Mount point. If you click the down arrow to the far right of Mount point, you should see the option for root "/", the forward slash. If you want additional partitions, you need to use the same process. If you want to use unallocated space, you would click the Add tab rather than change. This was the process on 10.10, don't know if it has changed for 11.04 but it should be similar.

This generally means you chose to manually setup partitions, but didn't assign mountpoint(s) from the drop-down box.
You'll need to tell us exactly what you did - and what version as per above.
This looks like a reasonable tutorial for 11.04. Simple works best.

See other ways to use linux that may be way easier than a dual boot system. Consider free Virtual machines. They can run your linux within windows. They can run many OS's as long as you have ram or wish to pause or store computers. See Virtualbox, VirtualPC, VMplayer and there is a other ones too.

It is really the most easy and safe way to run linux.

If you still want to run a dual boot then consider backup of your system before you get too far along maybe.