If you have a current and legit version of Win XP or Win Vista, you can upgrade via Microsoft's website at a cheaper price than purchasing the full version. Here's a link for you:
http://www.microsoft.com/windows/buy/default.aspx
If you don't have a legit version as mentioned above, your only way is to purchase the full version wich you can get at any BestBuy or most other places such as that or online. Careful with Ebay if you shop there... I've found a lot of counterfeits run on the auctions. Play safe and buy something you know is 100% legit.
As for Win 7 and UO... I have the 64bit Win7 and I have no problems what-so-ever. Only issues I have are with my actual internet provider. Cable connection but when there is deep snow outside or extreme cold (teens and lower temps) I disconnect at random times. When my signal is good though, I fly in game!
Win 7 is a little odd getting used to for me as I didn't have Vista, I had XP Pro prior to 7. If your used to Vista I'm told the transition in use is easier. I'm used to 7 now after a week of casual use and I LOVE it!
Good luck!