I've learned the hard way that D-Link products just aren't worth buying. I have a 802.11g router, that will overheat even in a 65 degree house with one computer connected to it, and just web browsing. It overheats, and then reboots itself. Apparently it's a well known condition with that particular model.
I also have an older 802.11b wireless card for a laptop, and that thing is also a turd. I killed about three D-Link routers in a row at work, trying to simply update the firmware... So, after all that, I won't buy another D-Link product again.
From what you describe, I might suspect that your router is at fault. If possible, borrow a different brand from someone for a couple of days, see if the problem persists. If it goes away, chuck the D-Link router, get something else (I'd recommend Linksys or Belkin myself). Otherwise, the other likely scenario is you have a spyware problem.
I have a Dell PC which is a Pentium III 750MHz with 640MB RAM. I'm running Windows XP Pro w/ SP2 on it and 2 user accounts (limited & Admin). It is setup with numerous shared folders. These folders are protected by user account specificity (e.g. limited user can't even access most of the shares). It is connected (via LAN) to a D-Link wireless router (802.11b). I have a total of 4 PCs connected to the router (1 LAN and 3 Wireless) with security (WEP) and MAC filtering.
My issue is that my PC seems to lose its connection to the internet and to the other computers after a short time. After a fresh boot in the morning or allowing to remain off for ~15 minutes the system can be accessed by other PCs with no problem. The issue comes that after some unknown event, the computer is able to access the internet but locks up quickly (e.g. is not able to fully load the next page) but can still load another webpage like www.yahoo.com but can't go much deeper into the website than that. With the file sharing, other computers can see the PC on the network and even access the list of shared drives I have setup, but are unable to go any deeper than this (e.g. can't access files on the shares). In order to fix the problem, the PC must be shutdown for a good 15 minutes before other PCs can access it again. This PC however can access other PC shares on the network with no issues until data transfer is necessary.
I have narrowed it down to the PC and not the router due to the latter statement that it can access other PCs without a problem. Also other PCs can access each other perfectly fine during the same time that the other PC cannot be accessed. The only error I can think of that I made is that during a recent spyware scan I accidentally removed the files needed for Remote Desktop connection needed on that PC. I have remote desktop connection setup for controlling my laptop, not the PC in question. Since then I have uninstalled the network card on the Dell PC and redoing the network shares which worked for about a week. I have also switched LAN ports on the router w/o success.
I know this is a lot of info, but someone please give me some advice other than reformatting my computer, this is the only way I know how to fix it.