I'm setting it as:
v=spf1 a mx ip4:xx.xxx.xx.xxx a -mx -all
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An SPF record depends completely on what you want to allow and block. We're just shooting in the dark if we don't know what you're trying to accomplish with this record.
Edit your question to include what systems you're intending to allow to send mail from your domain, and what their status is in regard to DNS records pointing to them.
That said, you likely do not want
Even if it is from Microsoft, but they have a wizard with explanation for people who want to implement a SPF record. Follow the step-by-step wizard, read carefully and you'll get your valid SPF record.
The wizard is located at http://www.microsoft.com/mscorp/safety/content/technologies/senderid/wizard/