TrueCrypt is a wonderful encryption package and best of all its free. Many people already use it to encrypt thumb drives and to setup encrypted file stores.
TrueCrypt has had system encryption (boot drive encryption) since version 5. This is a similar technology to Vista BitLocker. The entire boot drive is encrypted. Before your operating system loads you have to enter a password. This means you don't have to worry about the page file containing your encryption keys after shutdown.
I will be doing some traveling soon, and I was a little concerned about the safety of my data. You hear all kinds of news stories recently involving theft of laptops. With TrueCrypt you can do an easy in-place conversion of your existing system drive. It also allows you to convert it back. This convinced me to go ahead and give it a try after fully backing up my system.
I need a very responsive system for development, so the possible performance impact worried me. This of course leads to benchmarks. I used the hard drive benchmarks included in PC Wizard 2008.
Processor : AMD Turion 64 X2 Mobile TL-60 @ 2000 MHz
Ram : 2Gb
HDD : SATA 250 GB (5400 rpm)
Encryption Standard : AES (Best Performance)
As you can see there is a decent performance drop mainly on the buffered IO. CPU usage is higher during IO operations (encrypting and decrypting). Qualitatively I haven't noticed much of a difference except slightly on bootup. I would say TrueCrypt is a pretty good choice if you need a secure system, though you may want to have a decently powerful system. It does utilize multiple cores so that helps.
You can download TrueCrypt here.