a Clean install would involve FDISK, Format Install. Should this be a OEM machine like Packard Bell, Dell, Mesh etc then do you mean a Factory install was carried out ?
From installing you Windows, then you need to install the Motherboard Drivers, Graphics, Audio, etc.
Have you got the Modem cable in Line & not Phone on rear of PC.
Check the status of the ISP you use and see if there having any problems, if they are this could be the cause.
Have you recently upgrade or changed any hardware ?
Check your CD Drives, Floppy drive is working fine.
Then run a HDD Diagnostic Test. for the manufacture of your HDD.
Most importantly download the Windows Updates (don't install service Pack two just yet) until the problem is fixed.
Also check you Graphics Card/chip display properties, maybe its adjusted itself from a 3rd party software.