Just finished upgrading my neighbors HP Pavilion Slimline, here is what I done to it

Replaced the 500GB HDD with a 480GB SSD

Upgraded the memory to 8GB instead of 1GB

Installed Windows 10 Home instead of XP Professional

Re-applied the thermal paste to the CPU and GPU

Cleaned all the fans

He paid me well for the full upgrade ($130) he is a very generous man