You could have signal interference with your cordless phones.
Do you have cordless phones?
What frequency are they? 900mHz? 2.4GHz? 5.8GHz?
2.4GHz phones are operating at the same frequency as your wireless network. Just like radio stations that bleed over into another channel, you need to tweak your hardware a bit to transmit on one particular channel. Try changing the wireless channel on your router or change the channels on your phones (if they have that capability).
Which model router are you using?
Is your wireless card built-into the laptop or is it a separate PCMCIA card?
Is the card/router combo connecting at 802.1b (11mb) or 802.1g (54mb)?
Start with the simple steps like updating the firmware in your Linksys router. Download the file for your exact model and version from the Linksys website.
Then make sure your laptop has updated patches & drivers. Download updates from Microsoft's windowsupdate site AND check your laptop manufacturer's website for bios & driver updates.
If your wireless card is a separate PCMCIA card, check the manufacturer's website for an updated driver for the exact same model/version number.
If you update your software & firmware, you can eliminate additional testing with WEP/WPA & MAC address filtering.