I installed successfully virtualbox on slackware 13 and i am using windows xp as a guest os. But there's a problem: Everything's working fine (the network too) but i can't see the virtual machine ? I type ifconfig but no other adapter shows up except the usual ones. The network configuration of the guest os - windows xp says that 10.0.2.2 is used as default gateway and the actual ip is 10.0.2.15 - i pinged 10.0.2.2 from the guest, i pinged google and everything's fine but when i ping 10.0.2.2/15 from my host os - slackware, it's saying they are dead.
Why ? In the windows network config it says ips are assigned by dhcp. Is this the problem or what ?

It looks like you have the Windows client machine setup for Nat networking under VirtualBox. This uses a VirtualBox driver on the host to handle dhcp and nat so effectively the network address translations and address allocation is done internally on the host. If you want the guest OS to be visible to the host select bridged networking. If your host box gets it's ip via dhcp then set the guest similarly - it should get another IP via dhcp in the same subnet. This assumes the host network can cope with this,(if you get your IP's from an attached router then this is usually OK). The other way of setting up bridged networking is to have the guest statically assign an IP on the same subnet as the host i.e. set the guests IP address yourself. This still needs bridged networking to be chosen for the guest.

I tried bridge networking but again the same problem - i can ping from the guest and i can't ping from the host. I checked almost everything, and in ifconfig it doesn't show me any additional adapter.
How to properly configure the network between the guest and the host so i can access "remotely" the guest from my host via the network between them ?

A question first. Are you getting the guest's IP via DHCP or setting it statically? In either case make sure the guest and host are on the same subnet i.e. not the Nat supplied 10.0.2.0 subnet.
On the guest only existing adapters will be shown as the bridging/nat work is done internally by the VirtualBox drivers. You are loading the drivers aren't you?