Disadvantages of buying a computer from an original equipment manufacturer:
1) Likely get a cheap power supply
2) Other components are likely lower quality (such as motherboard)
3) A lot of original equipment manufacturer systems are not easy to upgrade (For example many systems don't have any space in the tower for an extra hard drive.)
4) No operating system disk. If you get a really bad virus or your hard drive goes you may have to spend money and buy another operating system installation disk
5) Junk Advertising software can also come pre loaded on your personal computer
Advantages of buying a computer from an original equipment manufacturer:
1) Comes pre-assembled, no headaches of having to build and work the bugs out of a system.