Hi and welcome to the team
Just leave it running in the background and it will be OK. The estimated time is just that - an estimated time. As you crunch your way through workunits that will gradually become more accurate as BOINC learns about your machine and adjusts the time accordingly.
If your machine gets too hot or too slow while you are using it, you can always adjust how much of your cpu it uses by going to Advanced/Preferences in the boinc client. I have boinc running on my laptop and have it set to use 50% of my cpu's and 50% cpu time so that it doesn't get too hot.
If you have an nvidia gpu, you can also use that, or if you have an ati card (58xx, 48xx, 38xx), you can use that for Milkyway@home for some serious point per day). There is an OCUK team for that as well
GPU crunching is a little bit more fiddly, and you have to turn it on or off when playing games as well, so if you are not too sure, just leave it for now and it can be something you can look at later if you like..
Seti shut their servers down from Tue to Fri, so you might want to set your cache to 3 days to give yourself enough work to last, it is set to 0.25 days by default - it depends on how long your rig is turned on for each day
got some stuff to do now, but I will post back later with instructions on how to install the optimised apps for seti if someone doesn't beat me to it as that will give your credit a nice increase for the same amount of effort