Hi, I think its great u want to try out Linux.
I suggest u try out Linux Mint 4.0, its based on Ubuntu but slightly easier.
http://www.linuxmint.com
I really suggest u use a torrent to download it to reduce chances of corruption other wise u can check it for corruption as well like they have stated.
Now for partitioning, lets assume u have a 120gb hard disk.
Vista requires u have a 40gb hard disk minimum. Create a new 40gb hard disk for Vista and install leaving the rest of the space unpartitioned.
After Vista is installed, put in the Linux Mint cd u have and reboot your computer using the cd, u will enter live cd mode which even though slightly slow will allow u to test out Linux and see whether it works well and if u like it or not.
Here comes the difficult part, after u choose to install then setting up partitions is what troubles most new users I think.
Linux requires 3 partitions by default.
Windows Vista uses the NTFS file system whereas Linux uses the ext3 file system currently.
You need to create one partition with a mount point of "/" using the ext3 file system. This is your root partition, this is where Linux will be installed. Make this of not more than 20gb, Linux will take up like about 4 or 5 but its good to have more space for reliability, stability, and in case later u want to install more software and updates.
Now 60gb of the hard disk is used and 60gb is left.
The second partition u need to create is with the mount point "/home" using the ext3 file system. This is the partition where all your stuff will go and u will have most access to this partition. This is like My documents in Windows. All your personal settings and stuff, your music, videos, etc go here. From its name u can tell its the Home partition meaning for the user.
The first partition we created was the root partition which is for the main administrator called root. In Linux there is only one root user account who can make system wide changes, its not recommended to run as root. Running as root user is like running as a default user in Windows Xp meaning if anyone attacks u then they can gain full control over your system just u like do.
Anyway, the Home partition should get 58gb meaning 2gb is left and I will explain what this is for.
The remaining 2gb should be mounted as "swap". This is ur swap partition, usually this is 2 times ur ram, but 2gb is usually enough. This partition is like extra ram. In case ur ram gets full then Linux uses this just like virtual memory in Windows, but Linux's way is faster. This is also required for power management options like suspend to ram and suspend to disk which is called hibernate in Windows.
So its really easy, one root partition, one home partition, and one swap partition.
I hope I helped, if u have any more questions then just email me if u want.
Edit: I just read your computer specs and it should work perfectly with Linux without any problems. For your hard disk just use enough space for Vista as much u think u require and give the rest to Linux. Give 16gb minimum to the root partition, 2gb to swap and the rest to the home partition. Their order should be like this:
/ = root
/home = home partition
swap = swap partition
Good Luck and take care!