To be clear, is this SLOWER than usual like you've had it for a long time now it's a lot worse?
If so, it sounds like a software issue. If so, my advice is:
1) Backup the C-drive if possible (Acronis True Image free for WD, DiscWizard for Seagate) to a secondary hard drive. Or at least backup your critical data like pictures etc.
2) RESTORE the backup image which should be on a hidden partition (may be started with an F-Key on startup but you'd need to look into that)
3) Get the updated DRIVERS for motherboard, video card.. (dell site should have software to manage that)
4) Reinstall programs.
OTHER:
If you had a spare hard drive laying around, you could unhook your current drive then install Windows 10 Technical Preview and see how your system performs. If it ran stably then it would indicate how Windows 8 would work (I update my dad's old laptop to Windows 8 and it worked far, far better. It had 2GB of system RAM as well).
I'm willing to bet since it's almost 10-years old you've got a lot of software issues built up over time. It's likely Windows 8 would make it work far better than replacing the drive with an SSD. Also, if you got an SSD you'd likely have to buy a new copy of Windows unless the backup Image would restore to a separate drive.