As the Dell Dimension 4600i is circa early Windows XP, it has 3 IDE drive ports and 1 SATA port for your drives. I know because I had one of those machines. Fortunately it died a few years back with a BIOS update corruption issue, and I don't have to deal with that any more. It too, used a Pentium 4 2.8 single core processor, and did not have hyper-threading technology built-in.
You are already looking at obsolete and ancient technology which is slower by a factor of at least 2 using IDE ports over SATA and Microsoft wants you to run Windows 10 on it? Not going to work well and nowhere near what one might expect due to old hardware and specifications.
You are fortunate it is still running. This is a good candidate for running some version of Linux, such as Ubuntu to use what usable life is still left in the hardware, not Windows, let alone running the latest version of avast!. Backwards compatibility can only go so far as far as Windows is concerned.
I'd get rid of the current windows 7 install and run Linux on it
but ensure all personal files are removed and backed up before installing Linux on it so you can at least have access to all your old files on the Dell once you do that. Linux is designed to run on older machines such as this one, and performance for what you need it for will be much better.
http://www.ubuntu.com/download/desktop You want 32-bit as this is a 32-bit machine.
http://www.ubuntu.com/download/desktop/install-ubuntu-desktopLinux Mint:
http://linuxmint.com/ Again, 32-bit as 64-bit will not run on your machine. Pentium 4 on the Dell is 32-bit and cannot run 64-bit. Try the Cinnamon version as it is best for the beginner.
http://lifehacker.com/5993297/ubuntu-vs-mint-which-linux-distro-is-better-for-beginners