For a unit that old, especially a retail HP, don't waste your money. If you simply cannot live without upgrading something, add 2gigs of RAM, but remember that you will have to purchase old memory from somewhere, and it will not substantially increase the computer speed. Sorry, but you can't make a thoroughbred race horse out of an old T-rex.
it does take a while to start up and can be sluggish sometimes
First, these retail units are never going to be as fast as a well-built home build unit.
Second, I am sure that there is a ton of garbage loading that has accumulated over the years. Run an elevated Hijack This scan and nuke every BHO and Toolbar listing you can find. There will probably be a dozen Microsoft Internet Explorer browser loaders at the top of the log. Nuke them all. Some HT may not be able to remove so you will have to remove them manually. There will be lots of "update helpers" that load on boot for programs, so go into the software settings and disable them. You can update your software manually when you want to, you don't need the "help" of these things. HP is notorious for packing lots of "helpful" overlay shells onto Windows that "make is easier" to perform the functions that Windows already has commands for. All of these can go. I do this all the time for client computers and they finally begin to run properly.
Also, remember that if you install a new OS (Operating System) you will need the mainboard and chipset drivers for that OS that will work on your motherboard, so do your homework and make sure HP has posted those upgraded drivers for download. Especially any peripherals such as video cards, etc. Retail manufacturers are notorious for not creating the upgraded drivers for old machines and newer OSs; after all, they sold the unit and any additional cost after that is a dead expense, even paying a coder to create upgraded drivers. Unfortunately.
BTW.....I will add the following for my self-protection because there will be a flurry of excited posters warning you not to use Hijack This: create a separate folder for it on the drive and run it from there. Do not remove any items that you don't know what they are, you can damage your OS. Have an updated image backup of your hard drive handy so you can restore if necessary; HT also keeps a backup of all removed items so you can restore if necessary. You can Google any file extensions listed that you are unsure about and find out what they are that way.
Also.....Linux is great, I agree, but if I am picturing your situation properly, your older Dad is not going to thank you for presenting him with an OS that is foreign to what he is used to. Plus he may have favorite programs he uses that will not run on Linux. He doesn't want to get fancy, he just wants to have his computer run and to leave him alone. The older I get the more I feel that way.