I was wrong; before leaving here, I used the Run command to find out I do in fact already have SP2 installed and found that MS only has an enhanced SP3 version available for download-to-save.
I just tested if you decline all the "offers" Microsoft does you can download the full SP3 to off-line install it's 316MB!
I'll need to use the online updater service directly to get the appropriate version on the Acer. I did get and save the Avast file from your link and have it on the Acer but see it has lots of features I probably don't need or want, particularly on that old system which won't be surfing far or often. I'll need to do a bit of 'research' do decide what parts I want to install.
If you won't surf much then MSE it's a better option for you. Avast free will not have all the options enable but will check allot more than MSE and this of course uses more resources.
The M Security Essentials link was apparently the wrong one for my system but because the files were so small, I downloaded over my line the 32 bit XP version from this link
All the links I have for MSE are about 10MB in size and the program will need to access the Internet to get the updates.
However, I now get a message partly through the installation about a filter package rollover manager file being missing. The message shows a link where I can get the file once connected to the internet, after I finish with Avast.
Most programs will require the Service Pack 3 installed, some will check that at install time others will give you errors like that.