Anytime you get multiple infections you should reformat - a single infection can be taken care of with Avast, but if you've got more than one then it's quite possible you have many others.
A quick way to ensure you never get caught badly is to separate partitions into a discrete Windows partition, containing the OS and all installed software, and a Data partition, containing all your personal folders, documents, data, and backups. Better still if they are separate physical hard disks, but you can also use RAID and partitions.
My system is set up for RAID mirroring on the two physical drives. I then have three partitions (Windows, Data, Swap). I also have an external hard disk with automated backup software that backs up the entire Data drive and a few folders on the Windows drive once a day. The external drive is actually a good option, and they're cheap (mine was $120 CAD for a 500 GB drive). Mine's USB-only, but some models also have eSATA ports which makes them even faster.