There are web-based services and desktop applications that you can use to snipe auctions for you.
With the desktop software(jbidwatcher, etc.), you have to leave your computer on and hope that your Internet connection is okay, your PC is running fast, and there are no temporary power outages that would cause your bid not to be placed.
With a web-based service(auctionsniper, Hidbid, etc.)you have to submit your eBay login information so that you can have bids placed for you. However, being web-based, the service would be totally independent from your computer, so it doesn't matter what type of computer that you have, a pc or Mac, if your computer is turned on or off, etc.
You just had to decide which you are more comfortable with.
If you go the web-based service route, hidbid.com is a
free ebay sniper I suggest using, but there are a plenty more that you could try.