Are dual OS or Multi-boot OS safe?

Safety is relative. Assuming you install Vista and XP on different partitions there is still some danger in one operating system clobering the other. A malfunctioning operating system can easily wipe out the entire disk (seen it happen though I’ve seen disks wiped out by human error a lot more often!).

If you want to be able to boot both operating systems with no danger one system corrupting the other than you have to use separate removable hard disks. There are kits to make your hard drive removeable so you can change hard drives between boots. A hard drive that isn’t installed can not be corrupted.

Another option is virtual machines. VMware server is available for free now and you can run a guest operating system in a virtual machine. Since I use VMware I no longer dual-boot. You will need sufficient RAM, hard disk, and fast enough a CPU but this is probably the best solution to needing to run multiple operating systems. A virtual maching crashing generally does not take down the entire system and in theory anyway a guest VM will not be able to corrupt the parent operating system