A few things:
1) Spybot Search & Destroy is a free anti-spyware application but doesnt protect against traditional viruses. I suggest downloading AVG Free Edition or Microsoft Security Essentials, both of which are completely free.
2) There is basic computer maintenance which should be performed regularly. If you run Windows 7, or Windows Vista, it is run automatically, but on Windows XP you need to run it yourself. You should run Disk Cleanup, then Disk Defragmenter. Then, from a command prompt, i suggest running a "chkdsk /r" and then if it asks you to run on next reboot say yes.
3) The figures you gave us are maximum data throughput figures but do not tell us anything about response time, latency, or ping(which are the biggest causes of lag).
4) Do you have DSL or U-Verse? Are you working wirelessly or are you connected with a cable? Are you using a router or connected to the modem/internet device directly?
5) Have you tried just rebooting the modem when it seems especially slow?