Not as long as you have a spare partition or drive to install Windows 7 on too. When Windows 7 installs it will see the Vista and XP installs and add them to the boot manager. But do a full backup first, just in case.
I have already done that in my computer. I have Windows XP SP2, Windows Vista and Windows 7. I have partitioned my hard drive in 4 80GB. The 3 partitions are for those operating system and the other one is for my back-up.
I have installed W7 and Office 07 full suit on my machine and it consumed 16 GB out of 30 GB provided so consider you have to provide least 20Gb for W7.