Can i put linux on a windows machine and keep windows

Several ways you could do this.

Run Linux on a Virtual Machine inside Windows, though this is pretty system intensive.
Install Linux to a different HDD or external drive, and boot off that.
Create a separate partition on your HDD, and install Linux to that. So you would have Linux and Windows on the same drive.

Its called dual booting. I know that ubuntu allows dual booting insert the USB or cd and go through the instillation process and there should be a option that allows you to set the partition and how big you want it to be. Then when its installed on your computers next start it should give you the option to run windows or ubuntu

do you know where i can get instructions?

If you just want to try ubuntu then it has the option to be installed through windows only. It installs just like any other software but after installation the Ubuntu option got automatically appear on the boot screen i.e dual booting. Also this is possible if this windows is 7.