the point you need to consider: at some time in the relatively near future (probably by the end of this year), Avast will no longer be supporting version 4.x. And without daily definition updates, your computer will immediately become unprotected.
If avast5 doesn't work well for you, then you'll need to consider alternative programs. With an older P4 processor, I don't know which one, if any, will be more palatable for you. Among free programs --- which you can test out (only one at a time!) and either keep or reject --- you might consider Microsoft Security Essentials (MSE) or Avira's Antivir. I realize this is the Avast forum. I, for one, am a happy avast user[who is responsible for the security of several extended-family members PCs]. I am mentioning these two alternatives, not to get into a quarrel with anyone here, but to offer you reasonable alternatives, on the basis that you have rejected avast5.