novice just bought a hp dm1 4108 looking help

hya I'm new here,pretty much a computing novice,just bought a 2nd hand 2month old hp pavilion dm1 4108,im just really into using it for youtubing,facebook,ebay hotmail, internet banking sorta thing also hoping2 down load music and some films in near future also, just wondering whats the best way to get it as fast n slick as possible ,i see things like upgradingmemory and some other stuff like getting rid of hp bloat ware stuff,but idnt know how really go about it,im on a budget ,dnt wana be spending more than maybe 100 dollars also,any ideas would be very much appreciated!! also its got 320gb hard drive,if i use maybe 1/2 -3/4 of it for music n films will it slow whole system down much and should i get an external hd?also is just using the free avast download for security recommended? ive had kaspersky before and it seemed to do the job is it worth paying 4it instead,as there is a norton trial thing with this but i heard norton slows things down alot? phew!!thats alota typing for me!lol any help idea's or suggestions would be very gratefully appreciated cheers francis!