Yes u can, i tried with a 2 ghz amd athlon xp and it works good, but your problem is the Ram, cause for parcial load l2off requires 4 Gb of ram and for full load 8 Gb, it takes a lot of RAM but each Gb of ram is like 500 players support so 4 gb is llike 2000 players support, your processor is not bad but your ram yes, so u cannot start it with that RAM,
And 2nd answer, yes u can do it with Windows Xp professional my is working on 1 like that, windows server is not a special os for l2off, is an OS made for SERVERS, im using on Win XP the Microsoft SQL Server 2008 Enterprise, so its not a problem.
U can see if your os is x64 or x86 (32 bits) if u press right click on the My Pc icon in the start menu and go to properties or control panel --> system. and u will find your properties, also u can use Everest, is a program that shows all your pc info.
have a nice day