Are altars able to spread crimson/corruption?
I had this perfect world where I basically had everything I needed. Then my friends and I battled the Wall of Flesh, I got the loot, used my magic mirror to get to my house, and it was all corrupted. After a while of trying to fix it I remembered I had an altar under my house. I always thought it would be a good thing having an altar near you so it's easier to craft special items, but I then thought it may be the reason why it spread to my house because of it becoming hard mode. Is this possible? I'm having to rebuild my house and it will take a while.
The altars do not spread corruption, unless you break them. When you break them they cause a small amount of corruption(or hallow) to appear randomly on the map somewhere. Starting hardmode however makes a LOT of corruption. It creates a whole diagonal stripe of corruption from the underworld all the way up to the top of the map (it also makes a stripe of hallow). It is likely that your house just happened to be in the way.
What happened
Firstly: The initial "V" of Hallow/Corruption/Crimson.
Most notably, notice the picture: 
(source: cursecdn.com)
This gamepedia article states that:
Immediately, two diagonal stripes of the new biomes generate, replacing large sections of the World in a "V" pattern that extends from the center of The Underworld all the way to the surface.
Things might have changed with 1.3, but most likely your house is near one of the "Sides" of the V.
Secondly: When you "smash" an alter you create small pockets of one or the other:
Upon smashing a Demon/Crimson altar with the Pwnhammer (Or more powerful tools later on in the game), there is a chance of a single block of your world turning Hallowed or Corrupt/Crimson in a random place, which given enough time, will spread to form new, large areas to deal with.
So the basic gist is that your house is in a "bad"/"unlucky" place. The biomes can spread - either with time or smashed altars+time - but if your house was "immediately" in a spot, it's all location.
What to do about it
Multiple methods exist to remove Hallow/Corruption. The easiest method is a Clentaminator - which spray changes an area into a biome of choice or cleanses an area.
The Clentaminator is the fastest means of spreading Hallow, Corruption, Crimson, and Mushroom Biome, or purifying biomes of Hallow, Corruption, or Crimson.
You can also clear a surrounding area - 3 blocks to stop the spread - and then remove all the corruption to clean the area or create an artificial biome at your home. You can create Jungle, Mushroom or similar biomes that will then remove the corruption biome.
Also, check your backup folder for a save from before the switch to hardmode for a "hard" reset back. This should be in the Terraria folder, under Documents -
C:\Users\$USERNAME\Documents\My Games\Terraria
Lastly, if desperate and map dependent, you can try to use TEdit - Its not been updated for 1.3 yet, but there is a chance that your world was made before. (backup your world before experimenting regardless)
As mentioned in comments, I've added these separate links to emphasize material - specifically Gamepedia - as a good spot for further research into Life-After-Hardmode. Plenty of good material on how stuff spreads, what to do about it, what not to do, etc:
http://terraria.gamepedia.com/Guide:Containing_the_Hallow/Corruption
http://terraria.gamepedia.com/Hardmode
http://terraria.gamepedia.com/Clentaminator