If I were in your shoes and I was for many years,  I would keep the old computer until it dies.  Just because its old doesn't mean that it as an expiration date stamped on it.  IC chips can last indefinitely providing you don't dunk the chassis in water or have a cat throw up on it.  Just blow  it out every year with a straw (let the spit dry for a couple of hours) and it will just fine.  

I still have a hard drive that I bought back in 1997 that works just fine.  Jan

PS use a good software cleaner like McAfee or even the built in MS Cleanmgr every couple of days, having junk on your hard drive will slow things down fast, also defrag the critter weekly.