Can I have both vista and windows 7 at the same time?

No, i don't believe you can install two windows. I can't explain you why because i am no tech expert but you can't do that. You could install another operating system but not two windows. You will have booting problems, and i am not even sure if the other would install over one windows. Both are windows and they both need the C drive and so i believe you can't do that. I am waiting for other's responses.

Yes you can have Windows 7 and Vista at the same time. You can create partitions on C:/ and then install both of them on different partitions.

i think you must move to win7 which is better than vista having all the functionalities of vista and also the speed. it also crashes less. so you must not use two OS. when they are almost same and on a common platform. they will take huge hard disk space. so be careful using both along with.

Yes defenately,you can use Win7 and Vista both in single machine,you only have to install it on different partition and as per driver requirements on your system. at this time i have been using Three system (XP,Vista,7) on my laptop.it was for experiment but its successfully running without any problem from last two months.