PPI (short for "Pre-Pulse Inhibition") is a feature absent in individuals afflicted with the mental disorders mentioned above.  Where the name comes from is that, in most individuals, a "warning shot" (i.e., pre-pulse) will prevent a second, larger stimulus from making them jump when it arrives moments later.  For example, if I say "Boo!" you might jump.  But if I say "boo" (normal speaking voice) and then give you the big scare a second later, odds are it won't spook you as much.  This is not the case in individuals with the disorders highlighted above, hence ADHD kids attend to the least distraction, autistic individuals are bothered by physical contact and deviations from routine, and so on.   I'm sure many of you reading this can think of examples of sensory stimuli that you could not filter out such that it either became painful or at least intrusive enough to keep you from concentrating.

Basically, it's like having a hangover or some forms of migraines in which sensitivity to stimuli is increased... only it's like that all the time.  Some people who have this to a large extent find that they are irritated by loud noises and/or bright lights more than their peers.  (I'm one of these people, incidentally.)

As for what you can do about it... that's a tougher problem.  If it is part of a larger condition that can be diagnosed (e.g., ADD/ADHD), doctors can then treat the big picture and (hopefully) deal with that component in connection with the rest.  However, there are certain modifications you can make yourself like avoiding clubs with loud music (try a coffee house instead?) and carrying a pair of ear plugs.  That doesn't look cool, but if you're going to be miserable otherwise, which would you prefer?  I would also suggest explaining what PPI is to your friends so they won't think you're being rude if you have to distance yourself from unbearable stimuli.

Here are some examples of a few of the things that particularly affect me:

Sound:

• telephones ringing
• chiming of a drinking glass (There was a scene with a wedding toast in "Mona Lisa Smile" that had me ready to kick in the tv set a couple days ago)
• alarm clocks (but who doesn't hate these?)
• applause
• loud music (the cymbals and snare of live drum kits in particular)
• doors that squeak in a really high pitch
• clanking plates (please use plastic ones)
• vacuum cleaners

Light:

• sunlight in the evening when the sun is close to the horizon (particularly when I'm driving and can't avert my glaze as easily)
• headlights of oncoming traffic (again, mainly when I'm driving)
• exposed bulbs such as on desk lamps if they're tilted upwards

Other senses:

• cold weather is miserable for me far worse than any of my peers
• spicy food is much more intense than others seem to experience it

There are other things as well, but those are the ones that come to mind at the moment.  Mind you, I'm not complaining so much as trying to illustrate this phenomenon.  In addition to frustrating you and making it hard for you to focus (which is already a problem), there are additional social implications to this.  Specifically, a study* found that problems with filtering out sensory input has the effect of making individuals appear more withdrawn and introverted

*Here's the reference:  Aron EN, Aron A. Related Articles, "Sensory-processing sensitivity and its relation to introversion and emotionality."  J Pers Soc Psychol.  1997 Aug;73(2):345-68.  A copy of the abstract is here: http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=9248053

Just lately, I've experienced the social awkwardness of this a couple times.  For example, I was at a friend's place for a party a couple months back.  He's a musician and had some friends over who he played with.  It's a small apartment, so everyone was in the living room while the guys played a few songs... with a drum kit right in the room.  Even though it was pretty cold outside that night, I spent a lot of my time out there.  I didn't want to stand inside with my fingers in my ears, so what else could I do?  Either way, it looks rude, but it can be unbearable otherwise.

Still, if you're with close friends, explain this to them ahead of time so it doesn't look like you're trying to indicate that you don't like the music or the place or whatever.  A perfectly reasonable comparison is that it's like a migraine or a hangover.  If they aren't sympathetic, ask them if they would like to go out to a club under those circumstances.  Odds are, they'll get what you're talking about since most people have had one or both of those conditions.

The problem is that sensory input comes in every aspect of one's interaction with the world.  It makes it difficult to interact with others when the medium of exchange (sight, sound, smells, etc.) can be tortuous.  That's hardly the last word on the subject.  I'm sure there is much better advice out there, but this is a framework for understanding what you're dealing with if this is a problem for you.