I read Chris Cleeland's answer and was appalled that his was the accepted answer. Let me first state that I used to be a bike mechanic, and I ride through inclement weather year round.
As another already stated, WD40 is useful for cleaning, but you should never use it on your drive-train (chain, freewheel, front cogs).
You wrote that you are concerned that: " My bolts, chain and discs for my brakes " are getting rusty.
- Chain rust is terrible for your bike and should be guarded against carefully. If your chain is already rusted, I recommend you go ahead and replace it immeidately. Rust on your chain will wear your casset and chainring, so it's not worth saving the money on replacing the chain only to have to replace the cassette later. I'll tell you how to care for you chain below.
- The bolts rusting is not really a big deal, although it's ugly, and will make maintenance harder. You can buy stainless-steel rust-free bolts and replace these for a minimal cost, which I do on all my bikes as soon as I see a bit of rust. You can also buy expensive titanium bling, but I consider that a waste of money. When you replace the bolts, pay attention to the torque (rotational force) and don't overtighten them. Do one at a time, especially on the headset, so you don't have to worry about changing the settings (if you loosen all the headset bolts at once, for example, you'll have to adjust the headset).
- I was surprised to hear you have rust on your disk brakes since I've never seen that before (I worked as a mechanic pre-disc brakes, and I never see rust on the bikes I currently maintain). I did a bit of googling around, and it seems to be a common problem. It's only cosmetic so you can safely ignore it -- any rust on the braking surface will quickly be scraped away when you brake. But since I hate rust, I would suggest rubbing it off with a scotch-brite pad -- but nothing which might leave an oily residue.
Okay -- now how to keep care of your bike:
- This is just opinion, but I suggest lightening up on the cleaning. A little dirt on an mtb just shows you actually ride. I generally just knock the mud off with a brush and water, so it's not a chore to do maintenance. I do try to keep the deraillers, pedals, chainring and cassette reasonably tidy (Brush and low-pressure hose is plenty). Keeping your brakes and drive train clean and well maintained is important however.
- I would never ever ever use a pressure washer or compressed air to clean a bike. Your components have well engineered seals to keep out water and dirt, but they are not designed to protect against a pressure washer or compressed air. You might make your bike look pretty, but degrade the mechanics. It can also drive dirt hard across your paint job scratching it up, so even on the static parts I wouldn't use it. A good scrub brush is much safer, and works just as well.
- How to maintain your drive train This is surprisingly easy, but few people do it right. First buy a lube that is appropriate to the weather you have. Your local bike shop can help you out there. Then always ensure that your chain has a light coat of oil. A light coat of lube means enough to lube it, but not enough to retain dirt. If you get black print of oil on your hand when you touch it -- you've oiled too much. Too much oil is as bad as too little, since oil grabs dirt, and dirt wears down your drive train. To oil your chain, just to the following: 1. put some oil on the chain. 2. hold a rag against the chain and cycle the chain through the rag until no more oil ends up on the chain. If you have a really dirty chain, and you get black ugly muck on the rag, you can repeat this process a few times. Do this regularly and it's super quick process. I suggest you never oil your chain without wiping it with a rag afterwards.
I generally oil and wipe my chain once every couple of weeks, and whenever I've done a really wet or muddy ride. Once in a while I'll pop the rear wheel off and clean the cassete if I notice it's getting gunky, but if you keep the chain well maintained this isn't often necessary. Same with the chainwheels.
As for WD-40 -- I'll use it to clean the chainwheels or cassette sometimes (wiping it away well afterwards), but I wouldn't ever spray in on the chain. It dilutes your lube, making it less effective. It's great on cables and bolts, but be careful not to let it slop over onto your disk brakes.