I've got a crappy old PC and it took what seemed like forever to install the new client so here is a guide to make it quicker for you.
*disclaimer : this worked for me may not for you .
1. Try and start the old launcher. You will get a popup message like 'full install required do you want to go to download site now?'
Answer = yes if you've not got the new client.v1.170 16/dec/2008.
Answer = no if you have.
2. Double click the new client exe from where you stored it on your hard drive. 'Please wait' should come up for what feels like an age. ghey.
3. Now I got the popup message 'client only for use after legend 3 update' wtf its already out? anyway click ok.
4. After a while my firewall alerts about the new client.exe (eg silkroad_global_offical_1.170.exe) trying to connect to the internet do I want it to.
Answer = Yes.
5. I gave it a couple of mins then double click the new client again.
'Please wait' again for ages again. GHEY. >:[
6. Now it says something like 'silkroad already installed do you want to delete and install?'
Answer = yes.
7. Now you will be asked the usual (where to install C:progfiles/silkroad etc.) Do the do.
8. Now it unpacks/installs the new client. Hoooray.

9. Now start the launcher. Lets play

.
no, wait, it has to update. dam dam dam u. ultimate gheyness now I've missed CTF. >:(
10. ok menz now you get big start button. WOOTS.
bai.