how much data do u have on ur pc ? 4 hrs is a hellava long time
download and use "ccleaner" to clean up ur junk n unreqd files. remov all (but the last) restore points using winxp diskcleaner. then run avast once using ur exising settings. aftr it u may if u wish change ur scan settings from 'thorough' to 'standard' and uncheck the scan 'archive files'.
winxp/sp2 fw shud be good enuff ! just make sure its on. only if u feel the need, the go for freeware za fw. i trust u have an anti-spy on ur pc. do install a good one like adaware, avg anti-spy or windows defender, and for the first time 'full scan' ur pc once in SAFE MODE with it.