System that old is probably clogged with dust: Remove fan, noting which way it blows so you can put it back the same way. DO NOT DISTURB HEATSINK. Clean dust from heatsink with vacuum cleaner hose and small brush. Clear dust from vents and fans including fan in graphics board and power supply. Reinstall fan. Start it up and note that each fan starts up quickly without excess noise. Replace any that make noise.
Get new internal desktop drive. Get a Western Digital Black SATA drive. 1 or 2 TB. DO NOT GET A SEAGATE. They don't last.
Clone drive if old drive still works. Use AOMEI
or the program that you can download from your new drive maker's website.
(Once you clone it and get it working you can use Disk Management to extend the partition to the whole drive.)
Is your Windows 7, 32 bit or 64 bit? If 32 RAM is good. If 64 you have 6 and could go to 8 but your PC used expensive RAM so probably not worth the cost.
HP does not support Win 7 on your PC which apparently came with XP but was Vista Certified. Most of the Vista drivers will work for Win 7 but go into Device Manager, View: Show Hidden Devices then look for any yellow flagged devices.
Right click on one and select properties then click on the Details tab. Change Property to Hardware IDs. Click on the top one then right click and copy. Paste that into a reply. Repeat for all yellow flagged devices. I can try to find drivers that will work.
If new install of Win 7, or if system is slow to check for updates you may want to running:
System Update Readiness Tool for Windows 7
This link is for 64 bit:
This one for 32 bit:
Once that runs then get
KB3083710 and KB3102810 if you do not have them