Please let me say that I am sorry for your loss.
The last version of Office to have the Office assistant as Office 2003. If you like Office 2000 and it does all you require of it then continue to use it, though there will be no more updates for it. Office 2003 is no longer available and current versions do
not use assistants but rely on help - much of which is on-line.
However Office 2000 is not compatible with Windows 7.
Office 2007 was (and still is) available to download, though new product keys are not available. If you purchased via a download, then you would have been sent a product key by e-mail. If you can recall the account used to purchase it you can also re-log into
that account to obtain it.
If it was installed on the old PC (the one with Windows XP) then you can recover the key from that key from the PC using Belarc Advisor -
http://www.belarc.com/free_download.htmland the download files can be obtained, using the key from http://www.microsoft.com/office/downloads/
As for a refund on Office 2010. If you have also purchased that product see http://www.microsoft.com/mscorp/productrefund/refund.mspx for eligibility criteria.