Can't say authoritatively anything about his being in the 17th Lancers . . . his name does not appear on either the Musters of the Service Troops or the Musters of the Depot troops at any time during the war, but I also recently was researching an officer in the Heavy Brigade who served in the 1st Dragoons in the Crimea, but his name is curiously absent from all Service Musters for that regiment. But I digress . . .
The Medal Rolls for the 28th Regiment of Foot list a Lieutenant James Williams who was entitled to a Crimean Medal with Alma and Inkermann Clasps on page 102 of the regimental muster rolls. In the listings for the Sebastopol Clasp, on page 134 he is listed as a Captain. The only page I can find listing entitlements to a Balaclava Clasp in the regiment is page 112, and only 3 NCOs and a number of privates show entitlement to that Clasp.
From Hart's Army List 1860: James Williams was appointed an Ensign by purchase on 21 Sep 1852; Lieutenant without purchase 29 Nov 1854; Cpt by purchase 25 Sep 1855. Note 14: Captain Williams served the Eastern campaign of 1854-55 including the battles of Alma and Inkerman, siege and fall of Sebastopol, and the affair in the Cemetery. Served in Turkey from Feb 1856 as aide-de-camp to Major General Smith, Commanding Osmanli Irregular Cavalry until the disbandment of that force (Medal with three Clasps, 4th Class of the Medjidie, and Lt-Col Turkish Army).
There may be more about him in later Hart's, which you can find by searching his name and Hart's Army list via Google Book Search (use the advanced search for greater options)