Scenario:
Customer: How much does a Vista Ultimate Retail version
cost?
Salesman: Do you know it costs a lot?
Customer: Yes, I do.
Salesman: Then why still want?
Customer: Flexibility and better support. Free 90 days support,
no worries over licensing issues.
Salesman: It's the same.
Customer: 
The salesman is right, but the customer is right as well...
between OEM Ultimate and Retail Ultimate, there's no difference
basically, in terms of functionality or whatsoever.
One wonders why if there's no difference, why the big price
difference?
Bear in mind, a retail version costs nearly 2X more than an OEM
version.
The answer lies in licensing and support. Licensing wise, a full
retail version allows you to transfer the license from one to
another without having MS bugging you that your license is
invalid.
For OEM, that's not the case. If you attempt such a stunt, MS
will declare it as invalid (simply put, pirated, regardless of
whether you paid for it).
In terms of support, a full retail version has better support.
You get free support from MS for 90 days. For OEM, there's no such
thing, because OEM means Original Equipment Manufacturer. Whoever
produced that machine, you call him/her and ask him/her to fix.
But if you build yourself, you call yourself???!!! 
Yes,
that's right.
Another option, is of course, pay and get it fixed. Business for
me. $100 per day. Maybe should change it to per hour and part
thereof. 1 hour and 5 minutes = $200. Easy money. 
Anyway, enough of my nonsense over money... there's one
important issue to take note when buying MS stuffs. MS doesn't
charge extra for nothing over the same product. There's always some
extras somewhere or legal issues that results in the extra
charges...
Don't be taken in by these salespeople... you will cry when you
bang into MS walls.