If you are new to PC's , know nothing/less about H/W, etc... then go for a branded PC like HP, Dell...
Coz buying a branded PC has a lot of advantage,first the QUALITY, & if you have any problems with your PC H/W or S/W (O.S), the Manufacturer (e.g. HP) will sort it out for you.
The only disadvantage is that you pay a lot for what you get, but again you get a lifetime service support(charged after warranty).
Otherwise Go for the Assembled one.