Has it been a few years since you've moved into your home? Is the wear and tear starting to show?
Here's where I would put my time and money if it was my place.
Move over mould and mildew
If your bathtub and shower smells musty and you've noticed mildew on the ceiling and on the caulking, you've got a problem. If it's just a matter of poor venting and air circulation, you could get off easy with the replacement of a properly vented higher power bathroom fan for under $100. If you suspect that water is leaking behind the shower and bathtub, you need to call in an expert. A leaking bathroom could mean disaster if it's not repaired properly. Spend the money and make sure it's fixed properly or you'll be spending twice the amount down the road.
Check the seals and air quality
If it's not in your budget to replace the doors and windows, you can make some improvements by re-caulking all of the seals around your home. I've seen homes with hardly any caulk to seal cracks which means that moisture can get into your home and create a ton of problems you don't want. If you have a lot of condensation on your windows in the winter, turn on your bathroom fan or run a dehumidifier to see if that works. If not, you can install an HRV unit (Heat Recovery Ventilator.) For homes with forced-air heating, the unit is connected to the heating system duct-work. The HRV will vent out the home's old air while drawing in outdoor air and filtering it throughout the house. For new homes, an HRV unit would cost $1,000 to $3,000.
Up on the rooftop
It seems that the roof gets all of the attention but the chimney is just as important and should really be cleaned and inspected every year. Since most people don't hop up on their rooftops to take a look around, common problems like mortar cracks between the bricks or on the crown of the chimney can let water seep into cracks and cause damage to your attic and ceilings. If you have a wood-burning fireplace, have your chimney cleaned to prevent a build-up of creosote and other combustibles to prevent potential fires and flare-ups. Remember that carbon monoxide is generated anywhere fuel is burned and if your chimney is cracked, the deadly fumes can be drawn back down into your home. I say spend the $150 and have your chimney professionally cleaned and inspected each year - especially if you use your fireplace more than three times a week in the cold weather.
Screw it and save
At a loss for what you can do yourself that doesn't cost a cent? Instead of dashing out to buy a new door handle or replace your kitchen hardware because it's loose, spend an hour with a screwdriver and tighten kitchen hinges, bathroom knobs, kitchen pulls and door handles. It means money saved and time well spent.