Also...if your a college student or KNOW someone who's in college...you can get a REALLY cheap discount through the college store, for example here at University at Buffalo, students get their copies for under $50 !!
But here is a lil arning, back when XP first came out, I went over to Ebay.com to buy it. I found one for $49.99. The buyer claimed is was brand new, still sealed in box. He even had homemade picyures of it sealed up, and it looked legit. So I brought it. When it arrived, it was in a regualr cd case, and on top of all that, it was a SamSung CD-R disc, and to make things worst, it had no liscense key nor did the cd work. The cd was blank.
Microsoft had a special running where by providing them a business name(ficticious) they sold me a full, sealed, legit copy of pro for $39.95 plus tax and shipping. Came to just about 50 bucks. They say one per customer, but when they ran the special again, I entered a different email address and ..lo and behold, I bought a second full, sealed, legit version of Pro for again, about 50 bucks delivered. Check it out..don't have the link now, but I'll look for it.