Hello Arnaldo and welcome to the forum.

Did you reboot after your installation?
1 GB is probably the minimal amount you should have, and
ideally you probably should have more if possible.
I need more information about your system:
1. What is your OS, 32 or 64-bit?
2. What version of Avast did you install? 5.0.677 is the latest version.
3. What other security software do you
currently have or did you have
in the past on this machine including antivirus (AV), firewall (FW), and other security programs, previous trial or pre-loaded security software? If they were in the past, how did you remove them (the vendor's uninstaller's tool or another way)?
4. Is it only slow when opening "My Computer" or other parts of your machine as well? Is your machine acting normally otherwise?