TVs generally use IR signals. It's common to use a PC running something like Girder to handle listening for and sending commands to IR devices. Other systems like a GlobalCache could also be used. But nothing exists that's cheap or easy to configure.
Fans, on the other hand, tend to use RF signals and I'm not aware of any PC controllers that know how to speak to them. They tend to be proprietary to the maker of the fan or the remote add-on.
By and large the answer is no but the exact details depend on your location.
Worldwide, most TVs and other AV devices use Infrared (IR) for control rather than RF.
In North America, X-10 uses 310MHz RF. Hardly anything else uses that frequency. Even if they did the X-10 RF remotes can only send the X-10 RF protocol.
In Europe (and many other regions) X-10 uses 433.92MHz for RF. Many (most?) RF devices also use this frequency (mandated by CE regulations). But, again, the X-10 RF remotes can only send the X-10 protocol.
X-10 does have multipurpose remotes which can send the X-10 RF codes and also send IR codes to AV devices.
There are some (usually high end @ $US1000+) RF capable programmable remotes (e.g. Philips Pronto) that can be used to control many other European RF devices. It's merely a matter of determining the codes and protocols.
However, there are roundabout ways to have the X-10 remotes control other devices by using a device (e.g. Ocelot) which can receive X-10 via the powerline and then send IR codes to control other devices.
There is also the just released Nokia 770 which has WiFi. It (or any WiFi capable PDA) can be used to send commands to someth>The X10 setup I have now can send RF commands to another X10
A few years back, X-10 made a universal remote that would send the RF signals out and would also control most home theatre devices via IR. You can generally find these things on eBay for around $5.00 plus shipping.