You can have your PC run as a router while also using it as a computer. However, wireless routers are very inexpensive these days (used, $10) so to avoid the hassle, I would instead adding a wireless router.
NOW
---phone cable to wall---[DSL Modem]===ethernet===PC
AFTER
---phone cable to wall---[DSL Modem]===ethernet======ethernet===PC
The router should do WPA or WPA security; if it only does WEP, get something else as WEP is not very secure.