Since there are many versions of .Net Framework available ,it is very difficult to download and install each of them seperately.For maximum compatibility you must install all the .Net Frameworks from version 2.0 and above.The all in one installer released by Microsoft is 231 mb.However you can download an alternative AIO installer(for XP) of size 34mb.
Just like how you can boot a computer using a bootable CD,you can also boot a computer using a bootable USB drive.
You can test a bootable USB Drive within Windows without restarting your computer.This is helpful when your motherboard does not support booting from a USB Drive.
You can easily test bootable iso without burning it to a CD and without restarting your computer using MobaLiveCD or Sun Virtual Box.
You can also test bootable CD/DVD in VirtualBox without restarting your computer.
With Windows 7, you now have the option to boot the OS from a VHD, a virtual disk file, instead of a partition. This way, you don’t have to reconfigure your hard drive partitions.
This feature is similar to Ubuntu’s Wubi installer which uses a similar concept to avoid reconfiguring hard drive partitions.
Basically, instead of installing windows 7 to your physical HDD by getting rid of Vista or even dual booting off another partition, you can install to a virtual HDD (in VHD format), and boot off the VHD so you actually get the hardware functionality to play with!!