As a Preston resident of 19 years, I can safely say stay away from the following areas:
Callon estate
Ribbleton
Larches/Savick
Frenchwood/areas of Avenham
Moor Nook
Brookfield
Holmeslack
Ingol
Decent/liveable areas:
Broadgate
Bamber Bridge
Deepdale
Fishwick
Docklands/Preston marina
Lostock Hall
Higher Walton
Plungington
Leyland
Hutton
Farington
Genuinely nice areas:
Walton-le-Dale
Fulwood
Penwortham
New Longton
Cottam
Longsands
Watling Street Road
Winckley Square (good luck)
Hoghton/Hoghton Bottoms
Gregson Lane
Coupe Green
Samlesbury/Samlesbury Bottoms
Ulnes Walton
Any part of the city centre north of the bus station
All things considered, if you don't find anywhere in the city centre, I think your best bet is Walton-le-Dale. A lot of the other places are genuinely lovely places, but they're a bit out in the sticks, so to speak, so commuting would be a problem.
Walton-le-Dale is a 10 minute bus journey from the city centre, has four decent pubs, three takeaways, an incredibly good Italian restaurant, an off-license, a post office, a hairdressers, a dentist, a doctor's, a chemist, a 24-hour garage, and a fairly sizeable retail complex called the Capitol Centre - Waitrose, PC World, McDonald's, Subway, Tesco, Maplin, Comet, TK Maxx, Homesense, Next, Boots, Vue Cinema, Frankie and Benny's, Pizza Hut and even a Virgin Active gym.
It sounds a lot bigger than it is; it's actually fairly quiet, just gets a bit busy at rush hour, but if you go out of the centre you can find some really stunning scenery - nice walks along the river, cycle routes etc, lots of fields and open spaces, old buildings/churches/bridges dotted around, quite an old feel to it. It is a really good mix and you're located ideally. Fifteen minute walk from Avenham and Miller Park too, through which you can actually get to the city centre on foot or by bike on a nice day. It's also really well served by bus. You'll never wait for more than ten minutes if you're going into town. There's also a Park & Ride which is only £1.20 return.
It's got its own recreational ground too which is generally tidy and well kept. Used by the local cricket and football teams. No scallies. So yeah, I would say try to find a room to rent in Walton-le-Dale. Fairly affluent area; I live in a terraced house, but some of the properties up near me regularly sell for £500k+. There is a church at the bottom of Church Brow (and one at the top, funnily enough) which has been converted into flats recently and I'm 90% sure one of those is To Let right now. I'll try to get the phone number if you're interested? It's in a nice location, right on the riverbank.
Generally a pleasant, homely, green area with friendly locals. Spent 17 happy, happy years here. As you have probably noticed, I may have a slight bias.