The best way to do it is have a friend that has L4D and loves you and trusts you to log out of steam and lend you a new password to access his steam account, and simply change the password back whenever your'e done.
The other way is to wait and see if there's a free L4D weekend or something like has been in the past.
OS: Windows Vista, XP or 2000
• CPU: 3.0 GHz P4, Dual Core 2.0 or AMD64X2 (or higher)
• RAM: 1 GB for XP / 2GB for Vista
• DISC DRIVE: DVD-ROM Drive
• HARD DRIVE: At least 7.5 GB of free space
• VIDEO: Direct X 9 compatible video card (Video card must be 256 MB or more and should be a DirectX 9-compatible with support for pixel shader 2.0)
• SOUND: Direct X9.0c compatible sound card
[quote name='soccerstud652']Not too great. I am running a Radeon 9600XT 128mb AGP card on 512 MB ram (P4). It runs everything but TF2 at the highest settings.[/QUOTE]
i've run left4dead on my similar HTPC (Radeon 9600 Pro 128mb PCIe, 512 ram, Pentium D) without problems, silky smooth, but at a low res