Most new PCs should start appearing with Office Start pre-installed - it is "free" on most machines. MS are using it to replace Works, but more as a path to sell upgrades to the full suite.
At the end of the day, it comes down to what features you need. If the PC comes with Office Starter installed, you don't have anything to lose. Openoffice is free anyway, so you can give it a try as well.
However, both come up short, if you need to exchange documents which use advance MS Office features (such as pivot tables or formatting in Word documents). I used OO.o for a few years, but always kept a copy of MS Office handy, on a test rig, for checking the formatting of documents, before sending them to customers (I was using Linux as my main workstation at the time), but it usually meant a couple of hours of correcting OO.o's poor interpretation of the MS formats...
If you are only doing documents for yourself and sending out PDFs or printed results, then OO.o is probably sufficient, if you need to send out documents in MS formats, then I'd get the Office 2010 Home & Student version...
Edit: Impress -> PowerPoint interchangability makes the Word -> Writer interoperability look like a model citizen (it isn't!). We had a presentation sent in by a customer, which the boss put on the computer in the meeting room (OpenOffice), the customer had done it in PowerPoint 2003. The lines pointed to the wrong boxes, text from inside boxes appeared outside the boxes, some lines were orphaned, it was a complete mess!
That persuaded him to invest in a copy of MS Office for proofing documents before sending them out (it was an advertising agency, so the incident was very embarassing and nearly lost them their oldest and biggest customer!).