My computer has become very slow over the past month. I have run all the usual scans, Malwarebytes, Super anti-spyware, and other on-line scans. Can anyone help?

My specs:
Computer Dell Dimension 2400
Intel Celeron CPU 2.20GHz, .99GB RAM
Windows XP Pro, SP 3
30GB Hard Drive, 16.7 KB Free
MS Security Essentials, AdAware
RoadRunner Cable