PC Upgrade suggestions!

I just installed Win7 64 bit + Outlook 2010 on my work pc and it runs so so so slow. I used to have XP and Office 2003 and it ran smoothly but now cannot identify what i’m missing from hardware 🙁 My system is as follows:
Hewlett-Packard HP Compaq dx2300 Microtower
3.00 gigahertz Intel Pentium D
ST3160815AS [Hard drive] (160.04 GB) — drive 0, s/n 6RX11A2V, rev 3.CHF, SMART Status: Healthy – 159.94 Gigabytes Usable Hard Drive Capacity
2816 Megabytes Usable Installed Memory (Slot ‘A0’ has 2048 MB – Slot ‘A1’ has 2048 MB)
NVIDIA GeForce 210 [Display adapter]

What shall I replace in order to have a convenient speed on my PC? Is it worth it or shall I go directly for a 100% replacement”?