Yup, there are various ways you can do it depending on price/picture quality you want.
A VIVO card will give you full uncompressed video - if you want to record at full resolution you will need about 26MB/s of disk space. Of course, you can compress on the fly to divx/mpeg depending on your processor.
A PVR card (Hauppauge for example) will do the mpeg enoding for you, allowing you to capture at a rate more like under 1MB/s.
S-video from your sky box is the best signal - composite will probably give you a poor signal (but an ok one if your not watching on a massive TV).
If you want to do this regularly and don't mind spending a bit, the best picture you will get is taking the digital feed from the sky+ box and capturing that - will cost you about £300 to get all the equipment ... but you can't get any better! (hunt for sky+ SDI modification).
Any signal coming out of the
Sky+ box is unencrypted and (unless you ripped out the sky+ hard drive) you would have trouble to get hold of the encrypted content.
Al