Boof -
an old HP computer
What is the actual model of computer, and how old is it? I know that a computer c. 2000 or older might not be able to recognise a drive larger than 80 GB. Or may recognise it but only at a capacity less than 80 GB.
The memory thing is interesting - it is quite possible that the system can only use a maximum of 512 MB RAM, in 2 x 256 DIMMS. Just because the 512 MB DIMM is reported correctly at boot time doesn't mean it would actually work, or all be addressable! I have seen a HP system report correctly that a system has 1.5 GB RAM, but then fail to boot an operating system - the max. supported RAM was 1 GB.
You should find definitive answers to all your questions on the HP support site - search for your PC model or serial number. You may have to hunt around a bit, but the information will be in there - somewhere! You should certainly establish what the max DIMM support is.
SD