Just wondering if you've tried CPUz to check the frequency of your CPU. Lonney suggested that, but I didn't see you post any results. If it's slower than Core 2 Duo, usually only several things can happen:
1) CPU is stuck at lowest frequency (Use CPUz to check: http://www.cpuid.com/softwares/cpu-z.html)
2) Your harddisk is broken or nearly broken, which leads to your OS spend most of time scanning bad sectors. (Use HD Tune to check)
3) If you are using an HDD, make sure you leave at least 10% free space for each partition, especially the system partition
4) You've installed sth, that keeps scanning the disk or using 100% CPU... (Or could be a trojan)
From your HD Tune report, it's likely to be the 2nd case. Reallocated event count should be 0 for a healthy disk, and any number larger than 0 means a pre-failure condition. Your count is terrible... (Paper from Google: http://research.google.com/archive/disk_failures.pdf) However, your status is "warning", and I am not sure if you can secure a free replace with your warranty. If you have a friend using T420, try swapping in his disk to observe if the problem is solved.
Please consult Lenovo support. They have an HDD scanning tool, and if that reports an error, you should be able to get a replacement. Select you model at http://support.lenovo.com/en_US/, then go to Diagnostics Software. I recommend that you use a bootable CD since your system is already slow.
