Buying a new PC(help)

I've decided recently to buy a new computer and I have no arguments with this. Step-Dad: Why do you need a new computer? This one does everything you need." See I need various reasons to go from Dell Inspiron E1405 to a Dell Xps 15z. I'm a web designer and so far all the programs I'm trying to run my video card or processor won't work on. Also my mother and father attend to downloading stuff taking up over 75Gbs of 100 for music pics and videos, and then keep to blaming it on me for my 3 programs...plus my pc is to slow to run some videos in high quality. It's also that thier pc is broke and down so they use my pc for "Facebook, Forums, and Youtube" I have been limited from as long as chores down n' grades good I can stay on as long, to 1 hour a day for their own selfishness. I need a new pc so they can have this one but they won't let me unless give a good 10-20 reasons. I needed to kinda let off some steam so half of this was not the question....

Short Question: Good reasons for going from Dell Inspiron E1405(Vista) to Dell XPs 15z(Windows7)


Thanks In Advance...

Specs on Dell Inspiron:
RAM: 1.00 Gb
SysType: 32-bit
OS: Vista
Year: 2004
Hardrive 100(11.5Gbs Free)